pgsql: Fix out-of-bound reads with ascii() for invalid multibyte charac

Fix out-of-bound reads with ascii() for invalid multibyte characters

This commit addresses two defects in this SQL function, the code
assuming that:
- The user-supplied string was long enough to contain a character of the
length implied by the first byte. It is possible to provide in input
data that was able to disclose a few bytes of server memory, allowing
out-of-bound reads.
- Specific bytes had values within the expected range, using a set of
assertions to validate them. The assertions could be triggered on
invalid input. These are replaced by tests and error reports.

Reported-by: Hcamael <baiyjrh(at)gmail(dot)com>
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Backpatch-through: 14
Security: CVE-2026-18024

Branch
------
REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/849da82105395fa4d06f877db8edc62158167941
Author: Michael Paquier <michael(at)paquier(dot)xyz>

Modified Files
--------------
src/backend/utils/adt/oracle_compat.c | 25 ++++++++++++++++++++-----
src/test/regress/expected/encoding.out | 19 +++++++++++++++++++
src/test/regress/sql/encoding.sql | 12 ++++++++++++
3 files changed, 51 insertions(+), 5 deletions(-)