UndefinedBehaviorSanitizer crash around slirp::ip_reass()

tag: v4.1.0-rc1

./configure --enable-sanitizers --extra-cflags=-O1

==26130==ERROR: UndefinedBehaviorSanitizer: SEGV on unknown address 0x000000000008 (pc 0x00000046d588 bp 0x7fff6ee9f940 sp 0x7fff6ee9f8e8 T26130)
==26130==The signal is caused by a WRITE memory access.
==26130==Hint: address points to the zero page.
    #0 0x0000561ad346d587 in ip_deq() at slirp/src/ip_input.c:411:55
    #1 0x0000561ad346cffb in ip_reass() at slirp/src/ip_input.c:304:9
    #2 0x0000561ad346cb6f in ip_input() at slirp/src/ip_input.c:184:18

I only had access to the last packet which isn't the culprit, I'm now seeing how to log the network traffic of the guest to provide more useful information.