pg_mblen_range, pg_mblen_with_len: Valgrind after encoding ereport.

pg_mblen_range, pg_mblen_with_len: Valgrind after encoding ereport.

The prior order caused spurious Valgrind errors.  They're spurious
because the ereport(ERROR) non-local exit discards the pointer in
question.  pg_mblen_cstr() ordered the checks correctly, but these other
two did not.  Back-patch to v14, like commit
1e7fe06c10c0a8da9dd6261a6be8d405dc17c728.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20260214053821.fa.noahmisch@microsoft.com
Backpatch-through: 14