SEGV crash in unwatch_fd()

My toy server crashed abruptly yesterday - no errors in the log. It is running the 9.13.5 master branch from 2019-01-09 rev f8965a55 plus miscellaneous patches unrelated to my recent KSK work. (https://gitlab.isc.org/fanf/bind9/commits/u/fanf2/patch)

Core file says:

#0  0x0000562957862065 in unwatch_fd (thread=0x7fbbdd09afc0, thread=0x7fbbdd09afc0, msg=
#1  internal_recv (sock=0x7fbbb4249580) at socket.c:3179
#2  process_fd (writeable=false, readable=true, fd=<optimized out>, thread=0x7fbbdd09b01
#3  process_fds (nevents=<optimized out>, events=0x7fbbdd09c010, thread=0x7fbbdd09b010)
#4  netthread (uap=0x7fbbdd09b010) at socket.c:3615
#5  0x00007fbbdb5c7494 in start_thread (arg=0x7fbbd3477700) at pthread_create.c:333
#6  0x00007fbbdb309acf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97

The other 18 threads are in various kinds of wait, some of which might be interesting...

I see @wpk was working in this area not long ago.

Let me know if you need any more info.