pgsql: Fix theoretical memory leaks in pg_locale_libc.c.

Fix theoretical memory leaks in pg_locale_libc.c.

The leaks were hard to reach in practice and the impact was low.

The callers provide a buffer the same number of bytes as the source
string (plus one for NUL terminator) as a starting size, and libc
never increases the number of characters. But, if the byte length of
one of the converted characters is larger, then it might need a larger
destination buffer. Previously, in that case, the working buffers
would be leaked.

Even in that case, the call typically happens within a context that
will soon be reset. Regardless, it's worth fixing to avoid such
assumptions, and the fix is simple so it's worth backporting.

Discussion: https://postgr.es/m/e2b7a0a88aaadded7e2d19f42d5ab03c9e182ad8.camel@j-davis.com
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/09d8c351744d3fdc7e1f72ab3a3b08b25e0c36f1

Modified Files
--------------
src/backend/utils/adt/pg_locale_libc.c | 30 +++++++++++++++---------------
1 file changed, 15 insertions(+), 15 deletions(-)