bug#58042: 29.0.50; ASAN use-after-free in re_match_2_internal

[Gerd Möllmann]

Can we come to a decision about what to do with probably_quit, based
what we know now?  The threads under this bug are a bit deep and
complicated, so I'd like to make this a bit more visible.

I think the problem has been analyized to be:

1. The re_matcher uses char* pointer P into data of string S.
2. The re_matcher uses maybe_quit
3. maybe_quit can call garbage_collect
4. garbage_collect can call Lisp (finalizers, redisplay)
(4a. That Lisp can again garbage_collect)
5. One of the GCs can relocate the string data of S in step 1.
6. P is then invalid.

Possible solution:

Inhibit GC in probably_quit, so that P remains valid.

Q: Should we do that?  And if so, when?