pgsql: Fix mb2wchar functions on short input.

Fix mb2wchar functions on short input.

When converting multibyte to pg_wchar, the UTF-8 implementation would
silently ignore an incomplete final character, while the other
implementations would cast a single byte to pg_wchar, and then repeat
for the remaining byte sequence. While it didn't overrun the buffer, it
was surely garbage output.

Make all encodings behave like the UTF-8 implementation. A later change
for master only will convert this to an error, but we choose not to
back-patch that behavior change on the off-chance that someone is
relying on the existing UTF-8 behavior.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/74ee636cc93d919c845e3e3ad3642e6366ce1802

Modified Files
--------------
src/common/wchar.c | 52 ++++++++++++++++++++++++++++++++--------------------
1 file changed, 32 insertions(+), 20 deletions(-)