null-pointer-dereference in a9gtimer
Description of problem:
The following log reveals it:

```
../hw/timer/a9gtimer.c:51:22: runtime error: member access within null pointer of type 'CPUState' (aka 'struct CPUState')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../hw/timer/a9gtimer.c:51:22 in
AddressSanitizer:DEADLYSIGNAL
=================================================================
==2624453==ERROR: AddressSanitizer: SEGV on unknown address 0x0000000002d0 (pc 0x55df9673422f bp 0x7fff7310e930 sp 0x7fff7310e8a0 T0)
==2624453==The signal is caused by a READ memory access.
==2624453==Hint: address points to the zero page.
    #0 0x55df9673422f in a9_gtimer_get_current_cpu /home/joey/repo/qemu/build/../hw/timer/a9gtimer.c:51:22
    #1 0x55df9673408c in a9_gtimer_this_write /home/joey/repo/qemu/build/../hw/timer/a9gtimer.c:246:14
    #2 0x55df97e00353 in memory_region_write_accessor /home/joey/repo/qemu/build/../system/memory.c:497:5
    #3 0x55df97dffa40 in access_with_adjusted_size /home/joey/repo/qemu/build/../system/memory.c:573:18
    #4 0x55df97dfd986 in memory_region_dispatch_write /home/joey/repo/qemu/build/../system/memory.c:1521:16
    #5 0x55df97ea8973 in flatview_write_continue_step /home/joey/repo/qemu/build/../system/physmem.c:2755:18
    #6 0x55df97ea81df in flatview_write_continue /home/joey/repo/qemu/build/../system/physmem.c:2785:19
    #7 0x55df97e7be4b in flatview_write /home/joey/repo/qemu/build/../system/physmem.c:2816:12
    #8 0x55df97e7b908 in address_space_write /home/joey/repo/qemu/build/../system/physmem.c:2936:18
    #9 0x55df96c8b041 in qtest_process_command /home/joey/repo/qemu/build/../system/qtest.c:559:13
    #10 0x55df96c8631d in qtest_process_inbuf /home/joey/repo/qemu/build/../system/qtest.c:811:9
    #11 0x55df96c955ae in qtest_read /home/joey/repo/qemu/build/../system/qtest.c:823:5
    #12 0x55df986c515d in qemu_chr_be_write_impl /home/joey/repo/qemu/build/../chardev/char.c:214:9
    #13 0x55df986c5219 in qemu_chr_be_write /home/joey/repo/qemu/build/../chardev/char.c:226:9
    #14 0x55df986d0d25 in fd_chr_read /home/joey/repo/qemu/build/../chardev/char-fd.c:72:9
    #15 0x55df980dd58c in qio_channel_fd_source_dispatch /home/joey/repo/qemu/build/../io/channel-watch.c:84:12
    #16 0x7f76346edc43 in g_main_context_dispatch (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x55c43)
    #17 0x55df98a66319 in glib_pollfds_poll /home/joey/repo/qemu/build/../util/main-loop.c:287:9
    #18 0x55df98a64c53 in os_host_main_loop_wait /home/joey/repo/qemu/build/../util/main-loop.c:310:5
    #19 0x55df98a6481c in main_loop_wait /home/joey/repo/qemu/build/../util/main-loop.c:589:11
    #20 0x55df96c9c807 in qemu_main_loop /home/joey/repo/qemu/build/../system/runstate.c:796:9
    #21 0x55df98048c6c in qemu_default_main /home/joey/repo/qemu/build/../system/main.c:37:14
    #22 0x55df98048cb7 in main /home/joey/repo/qemu/build/../system/main.c:48:12
    #23 0x7f7633e29d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #24 0x7f7633e29e3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #25 0x55df95222d34 in _start (/home/joey/repo/qemu/build/qemu-system-aarch64+0x2773d34)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/joey/repo/qemu/build/../hw/timer/a9gtimer.c:51:22 in a9_gtimer_get_current_cpu
==2624453==ABORTING
```
Steps to reproduce:
```
cat << EOF | /home/joey/repo/qemu/build/qemu-system-aarch64 -display \
none -machine accel=qtest, -m 512M -machine npcm750-evb -qtest stdio
writel 0xf03fe20c 0x26d7468c
EOF
```