In fact this crashes in normal (not embedded) run also.
  The problem is in the memory mapping. Handling the ha_myisammrg::extra(MMAP)
  the MERGE engine tries to mmap all the tables it unites.
  Though some can be empty and then in the mi_dynmap_file()
  we call the my_mmap(0). Normally this call returns MAP_FAILED,
  but not on FreeBSD. There it returns like a 'normal' value,
  and after the consequitive munmap systems gets unstable and
  crashes on some system call later.

per-file comments:
  storage/myisam/mi_dynrec.c
Bug #47139      Test "merge" crashes in "embedded" run
    don't try to mmap zero-length area, just return at once.