bird 3.1.4 Assertion 'heea' failed at nest/rt-table.c:4120

[Matt Corallo]

Thanks for all the work on bird3. It looks like 3.1.4 is much more stable than past releases, 
however I still get a crash if I start it while a routing table that bird is responsible for has 
stale entries left over from a past run which crashed. I now see

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo at entry=6, no_tid=no_tid at entry=0)
     at ./nptl/pthread_kill.c:44
#1  0x00007ffff7dd99ff in __pthread_kill_internal (threadid=<optimized out>, signo=6) at 
./nptl/pthread_kill.c:89
#2  0x00007ffff7d84cc2 in __GI_raise (sig=sig at entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007ffff7d6d4ac in __GI_abort () at ./stdlib/abort.c:73
#4  0x000055555566044c in bug (msg=msg at entry=0x555555670ee0 "Assertion '%s' failed at %s:%d") at 
sysdep/unix/log.c:412
#5  0x00005555555ebd92 in rt_next_hop_update_rte (old=0x7fffc74f2038, new=0x7ffff6a5d160) at 
nest/rt-table.c:4120
#6  rt_next_hop_update_net (tab=<optimized out>, ni=<optimized out>, n=<optimized out>) at 
nest/rt-table.c:4357
#7  rt_next_hop_update (_tab=<optimized out>) at nest/rt-table.c:4579
#8  0x00005555555ac0a4 in ev_run_list_limited (l=l at entry=0x5555557da288, limit=4294967294, 
limit at entry=4294967295)
     at lib/event.c:338
#9  0x000055555565d64c in birdloop_run (_loop=0x5555557da1b0) at sysdep/unix/io-loop.c:1836
#10 0x00005555555ac0a4 in ev_run_list_limited (l=0x5555558cce78, limit=4294967283, 
limit at entry=4294967295) at lib/event.c:338
#11 0x000055555565b40f in bird_thread_main (arg=0x5555558cd0d0) at sysdep/unix/io-loop.c:952
#12 0x00007ffff7dd7b7b in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:448
#13 0x00007ffff7e557b8 in __GI___clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78