pgsql: Fix encoding length for EUC_CN.

Fix encoding length for EUC_CN.

While EUC_CN supports only 1- and 2-byte sequences (CS0, CS1), the
mb<->wchar conversion functions allow 3-byte sequences beginning SS2,
SS3.

Change pg_encoding_max_length() to return 3, not 2, to close a
hypothesized buffer overrun if a corrupted string is converted to wchar
and back again in a newly allocated buffer. We might reconsider that in
master (ie harmonizing in a different direction), but this change seems
better for the back-branches.

Also change pg_euccn_mblen() to report SS2 and SS3 characters as having
length 3 (following the example of EUC_KR). Even though such characters
would not pass verification, it's remotely possible that invalid bytes
could be used to compute a buffer size for use in wchar conversion.

Security: CVE-2026-2006
Backpatch-through: 14
Author: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/df0852fe037246289cc00b4d36da6c1f25ff5844

Modified Files
--------------
src/common/wchar.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)