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
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/642b0b69b0638261fe81dfd9ca3b92927496f1cb

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