pgsql: ltree/crc32.c: fix fragile code.

ltree/crc32.c: fix fragile code.

Explicitly make space for the NUL when casefolding.

No known bug in the previous code, because the previous buffer (size
12) was more than large enough for folding any codepoint with enough
room left for a NUL. The builtin provider's limit is 7; ICU's limit
seems to be 7 also; and libc always does 1:1 mappings so the real
limit is MAX_MULTIBYTE_CHAR_LEN + 1 (size 5).

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reviewed-by: Rithvika Devisetti <devisettirithvika(at)gmail(dot)com>
Discussion: https://postgr.es/m/2cfbc37ae8deccd3825e36b7f31c391cbf8ff9b8.camel@j-davis.com
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8fa803dd695073a6fad49c49878b46783ac2f207

Modified Files
--------------
contrib/ltree/crc32.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)