Crash in bidi.c in Haiku port

[Po Lu]

Recently I experienced this abort twice in my Haiku port when
redisplaying a compilation buffer.

The abort happens at line 3363 of bidi.c, in this snippet of
bidi_find_other_level_edge:

      /* If we are at end of level, its edges must be cached.  */
      if (end_flag)
        emacs_abort ();

I can't seem to reproduce it reliably at all, and I haven't managed to
reproduce it in upstream Emacs, but if someone knows what might be wrong
here it would be appreciated.

Unfortunately, I haven't been able to save a core dump, as GDB
generate-core-file doesn't work on Haiku.

If you need any more information to debug this, let me know.  TIA.