pgsql: Fix overflows with ts_headline()

Fix overflows with ts_headline()

The options "StartSel", "StopSel" and "FragmentDelimiter" given by a
caller of the SQL function ts_headline() have their lengths stored as
int16. When providing values larger than PG_INT16_MAX, it was possible
to overflow the length values stored, leading to incorrect behaviors in
generateHeadline(), in most cases translating to a crash.

Attempting to use values for these options larger than PG_INT16_MAX is
now blocked. Some test cases are added to cover our tracks.

Reported-by: Xint Code
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Backpatch-through: 14
Security: CVE-2026-6473

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/62ad262661ed92b0fbd43e8b7bbd1e0e38cbb9c0
Author: Michael Paquier <michael(at)paquier(dot)xyz>

Modified Files
--------------
src/backend/tsearch/wparser_def.c | 24 +++++++++++++++++++++---
src/test/regress/expected/tsearch.out | 10 ++++++++++
src/test/regress/sql/tsearch.sql | 8 ++++++++
3 files changed, 39 insertions(+), 3 deletions(-)