tests: add a reproducer for an infinite loop in ndisc_handle_datagram

tests: add a reproducer for an infinite loop in ndisc_handle_datagram

=0  ndisc_router_parse (rt=0x60d000000110) at ../src/libsystemd-network/ndisc-router.c:126
=1  0x000055555558dc67 in ndisc_handle_datagram (nd=0x608000000020, rt=0x60d000000110) at ../src/libsystemd-network/sd-ndisc.c:170
=2  0x000055555558e65d in ndisc_recv (s=0x611000000040, fd=4, revents=1, userdata=0x608000000020) at ../src/libsystemd-network/sd-ndisc.c:233
=3  0x00007ffff63913a8 in source_dispatch (s=0x611000000040) at ../src/libsystemd/sd-event/sd-event.c:3042
=4  0x00007ffff6395eab in sd_event_dispatch (e=0x617000000080) at ../src/libsystemd/sd-event/sd-event.c:3455
=5  0x00007ffff6396b12 in sd_event_run (e=0x617000000080, timeout=18446744073709551615) at ../src/libsystemd/sd-event/sd-event.c:3512
=6  0x0000555555583f5c in LLVMFuzzerTestOneInput (data=0x6060000000e0 "\206", size=53) at ../src/fuzz/fuzz-ndisc-rs.c:422
=7  0x0000555555586356 in main (argc=2, argv=0x7fffffffe3d8) at ../src/fuzz/fuzz-main.c:33