Fix small memory leaks in GUC checks

Fix small memory leaks in GUC checks

Follow-up commit to a9d58bfe8a3a.  Backpatch down to v16 where
this was added in order to keep the code consistent for future
backpatches.

Author: Tofig Aliev <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/bba4313fdde9db46db279f96f3b748b1@postgrespro.ru
Backpatch-through: 16