pgsql: Fix potential buffer overrun in regexp match/split functions.

Fix potential buffer overrun in regexp match/split functions.

setup_regexp_matches() sizes the buffer used to convert matched
substrings back from pg_wchar form at the smaller of maxlen*eml and
the original string's byte length, on the assumption that such a
conversion cannot produce more bytes than the string it came
from. That assumption holds only for validly encoded input. But
pg_mb2wchar_with_len() silently accepts bytes that are invalid in the
database encoding, turning each such byte into one pg_wchar, and
converting that back can take more bytes than the input did. A string
made of such bytes therefore overruns the conversion buffer by up to
its own length, corrupting the following memory. regexp_match(),
regexp_matches(), regexp_split_to_table() and regexp_split_to_array()
are all affected.

Fix by dropping the tighter bound and always allocating maxlen*eml + 1
bytes.

Reported-by: Francesco Verardi <frevadiscor89(at)gmail(dot)com>
Author: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Backpatch-through: 14
Security: CVE-2026-14664

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e91dcfccaaa827f2907307e12891eeb131629948
Author: Masahiko Sawada <msawada(at)postgresql(dot)org>

Modified Files
--------------
src/backend/utils/adt/regexp.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)