pgsql: Don't leak malloc'd strings when a GUC setting is rejected.

Don't leak malloc'd strings when a GUC setting is rejected.

Because guc.c prefers to keep all its string values in malloc'd
not palloc'd storage, it has to be more careful than usual to
avoid leaks. Error exits out of string GUC hook checks failed
to clear the proposed value string, and error exits out of
ProcessGUCArray() failed to clear the malloc'd results of
ParseLongOption().

Found via valgrind testing.
This problem is ancient, so back-patch to all supported branches.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5058e95a6ef99e03dd5e41c9cda531cbed721d53

Modified Files
--------------
src/backend/utils/misc/guc.c | 71 +++++++++++++++++++++++++++++---------------
1 file changed, 47 insertions(+), 24 deletions(-)