Underflow in xlnx_dp_aux_pop_tx_fifo()
Description of problem:
Pop from s->tx_fifo but s->tx_fifo has zero element.
Steps to reproduce:
```
export QEMU=/path/to/qemu-system-aarch64

cat << EOF | $QEMU \
-machine xlnx-zcu102 -monitor none -serial none \
-display none -nodefaults -qtest stdio
writel 0xfd4a0100 0x19c4406f
EOF
```
Additional information:
```
+ DEFAULT_INPUT_MAXSIZE=10000000
+ ./qemu-videzzo-aarch64-target-videzzo-fuzz-xlnx-dp -max_len=10000000 -detect_leaks=0 ./crash-c15714102f0b894dea5c22f38852311567380926.minimized
==14660==WARNING: ASan doesn't fully support makecontext/swapcontext functions and may produce false positives in some cases!
INFO: found LLVMFuzzerCustomMutator (0x55db5cf9b840). Disabling -len_control by default.
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 1977030529
INFO: Loaded 1 modules   (618603 inline 8-bit counters): 618603 [0x55db600fa000, 0x55db6019106b), 
INFO: Loaded 1 PC tables (618603 PCs): 618603 [0x55db5f788d60,0x55db600f9410), 
./qemu-videzzo-aarch64-target-videzzo-fuzz-xlnx-dp: Running 1 inputs 1 time(s) each.
INFO: Reading pre_seed_input if any ...
INFO: Executing pre_seed_input if any ...
Matching objects by name , *.core*, *.v_blend*, *.av_buffer_manager*, *.audio*
This process will fuzz the following MemoryRegions:
  * xlnx.v-dp.core[0] (size 3b0)
  * xlnx.v-dp.v_blend[0] (size 1e0)
  * xlnx.v-dp.audio[0] (size 50)
  * xlnx.v-dp.av_buffer_manager[0] (size 238)
This process will fuzz through the following interfaces:
  * clock_step, EVENT_TYPE_CLOCK_STEP, 0xffffffff +0xffffffff, 255,255
  * xlnx.v-dp.core, EVENT_TYPE_MMIO_READ, 0xfd4a0000 +0x3b0, 4,4
  * xlnx.v-dp.core, EVENT_TYPE_MMIO_WRITE, 0xfd4a0000 +0x3b0, 4,4
  * xlnx.v-dp.v_blend, EVENT_TYPE_MMIO_READ, 0xfd4aa000 +0x1e0, 4,4
  * xlnx.v-dp.v_blend, EVENT_TYPE_MMIO_WRITE, 0xfd4aa000 +0x1e0, 4,4
  * xlnx.v-dp.av_buffer_manager, EVENT_TYPE_MMIO_READ, 0xfd4ab000 +0x238, 4,4
  * xlnx.v-dp.av_buffer_manager, EVENT_TYPE_MMIO_WRITE, 0xfd4ab000 +0x238, 4,4
  * xlnx.v-dp.audio, EVENT_TYPE_MMIO_READ, 0xfd4ac000 +0x50, 1,4
  * xlnx.v-dp.audio, EVENT_TYPE_MMIO_WRITE, 0xfd4ac000 +0x50, 1,4
INFO: A corpus is not provided, starting from an empty corpus
#2      INITED cov: 3 ft: 4 corp: 1/1b exec/s: 0 rss: 488Mb
Running: ./crash-c15714102f0b894dea5c22f38852311567380926.minimized
aarch64: xlnx_dp_aux_pop_tx_fifo: TX_FIFO underflow
==14660== ERROR: libFuzzer: deadly signal
    #0 0x55db5837410e in __sanitizer_print_stack_trace /root/llvm-project/compiler-rt/lib/asan/asan_stack.cpp:86:3
    #1 0x55db582c2d81 in fuzzer::PrintStackTrace() /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerUtil.cpp:210:38
    #2 0x55db5829bcb6 in fuzzer::Fuzzer::CrashCallback() (.part.0) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:236:18
    #3 0x55db5829bd82 in fuzzer::Fuzzer::CrashCallback() /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:208:1
    #4 0x55db5829bd82 in fuzzer::Fuzzer::StaticCrashSignalCallback() /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:207:19
    #5 0x7f98a612541f  (/lib/x86_64-linux-gnu/libpthread.so.0+0x1441f)
    #6 0x7f98a5f3700a in __libc_signal_restore_set /build/glibc-SzIz7B/glibc-2.31/signal/../sysdeps/unix/sysv/linux/internal-signals.h:86:3
    #7 0x7f98a5f3700a in raise /build/glibc-SzIz7B/glibc-2.31/signal/../sysdeps/unix/sysv/linux/raise.c:48:3
    #8 0x7f98a5f16858 in abort /build/glibc-SzIz7B/glibc-2.31/stdlib/abort.c:79:7
    #9 0x55db583a465a in __wrap_abort /root/videzzo/videzzo_qemu/qemu/build-san-6/../tests/qtest/videzzo/less_crashes_wrappers.c:24:12
    #10 0x55db58cce4d8 in xlnx_dp_aux_pop_tx_fifo /root/videzzo/videzzo_qemu/qemu/build-san-6/../hw/display/xlnx_dp.c:476:9
    #11 0x55db58cc9ee7 in xlnx_dp_aux_set_command /root/videzzo/videzzo_qemu/qemu/build-san-6/../hw/display/xlnx_dp.c:524:22
    #12 0x55db58cc6a92 in xlnx_dp_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../hw/display/xlnx_dp.c:800:9
    #13 0x55db5bf4eec3 in memory_region_write_accessor /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/memory.c:492:5
    #14 0x55db5bf4e801 in access_with_adjusted_size /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/memory.c:554:18
    #15 0x55db5bf4d126 in memory_region_dispatch_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/memory.c:1514:16
    #16 0x55db5bfdb2de in flatview_write_continue /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/physmem.c:2825:23
    #17 0x55db5bfc941b in flatview_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/physmem.c:2867:12
    #18 0x55db5bfc8ed8 in address_space_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/physmem.c:2963:18
    #19 0x55db583b40cb in qemu_writel /root/videzzo/videzzo_qemu/qemu/build-san-6/../tests/qtest/videzzo/videzzo_qemu.c:1088:5
    #20 0x55db583b2544 in dispatch_mmio_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../tests/qtest/videzzo/videzzo_qemu.c:1229:28
    #21 0x55db5cf971ff in videzzo_dispatch_event /root/videzzo/videzzo.c:1122:5
    #22 0x55db5cf8e57b in __videzzo_execute_one_input /root/videzzo/videzzo.c:272:9
    #23 0x55db5cf8e450 in videzzo_execute_one_input /root/videzzo/videzzo.c:313:9
    #24 0x55db583bb10c in videzzo_qemu /root/videzzo/videzzo_qemu/qemu/build-san-6/../tests/qtest/videzzo/videzzo_qemu.c:1504:12
    #25 0x55db5cf9bae2 in LLVMFuzzerTestOneInput /root/videzzo/videzzo.c:1891:18
    #26 0x55db5829c826 in fuzzer::Fuzzer::ExecuteCallback(unsigned char*, unsigned long) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:594:17
    #27 0x55db5827f454 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:323:21
    #28 0x55db5828a3fe in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char*, unsigned long)) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:885:19
    #29 0x55db582769e6 in main /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:30
    #30 0x7f98a5f18082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
    #31 0x55db58276a3d in _start (/root/bugs/metadata/xlnx_dp-06/qemu-videzzo-aarch64-target-videzzo-fuzz-xlnx-dp+0x3291a3d)

NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
MS: 0 ; base unit: 0000000000000000000000000000000000000000
0x1,0x9,0x0,0x1,0x4a,0xfd,0x0,0x0,0x0,0x0,0x4,0x0,0x0,0x0,0x6f,0x40,0xc4,0x19,0x0,0x0,0x0,0x0,
\x01\x09\x00\x01J\xfd\x00\x00\x00\x00\x04\x00\x00\x00o@\xc4\x19\x00\x00\x00\x00
```