pgsql: Fix ILIKE patterns with non-ASCII bytes in single-byte encodings

Fix ILIKE patterns with non-ASCII bytes in single-byte encodings.

Commit 1e493158d3d changed the MATCH_LOWER implementation of GETCHAR()
from SB_lower_char() to use pg_ascii_tolower(). The latter returns an
unsigned char; but the local variable in C_IMatchText was a char.

For single-byte encodings and the C locale, that could lead to a
broken signed/unsigned comparison. For example, the pattern
U&'%\00C6' would never match.

Fix by casting the result of pg_ascii_tolower() explicitly to char.

Reported-by: Greg Burd <greg(at)burd(dot)me>
Reviewed-by: Greg Burd <greg(at)burd(dot)me>
Discussion: https://postgr.es/m/_GW2V-rd0QW4BTguAJriA9XVmjOn2mR3-YeAZr9ZU6EtlLvYPEm_r_fV5wGKzSNMzboj_nqn3fk72AYAvOKzEnPlCCHteaYrgESrIhPg6l8=@burd.me
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e24685619daed171fe28d0857abfb92deac6e5a8

Modified Files
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src/backend/utils/adt/like_match.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)