bug#56108: 29.0.50; ASAN use-after-free in re_match_2_internal

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Date: Mon, 20 Jun 2022 16:07:55 +0200
> 
> FWIW, here is another non-reproducible crash with ASAN.
> 
> In short, shrink_regexp_cache realloc'd something leading to a malloc +
> free, and something is still holding a pointer the old memory.  Or so it
> looks to me.

I don't understand why some callers of compile_pattern mark the cache
entry as busy, but some others don't.  If a cache entry that is in use
is not marked as busy, then any GC can decide to shrink the cache by
freeing that entry.