And "p history_offset"? Should be 0.

Nope - it is 2:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff79b6a0b in previous_history () at /build/readline6-RKA9OI/readline6-6.3/history.c:185
185 /build/readline6-RKA9OI/readline6-6.3/history.c: No such file or directory.
(gdb) p history_offset
$1 = 2
(gdb) p *(the_history[2])
Cannot access memory at address 0x10

We are up to something! :-)

-c

PS: I did: Invoke birdc, „show proto\n“, either ^P or ↑ → 💣