Fix minor memory leak in pg_dump
Fix minor memory leak in pg_dump

A query used to read default ACL information from the catalogs did not
free a set of PQExpBuffer.

Oversight in commit e2090d9, so backpatch down to 9.6.

Author: Jie Zhang
Reviewed-by: Sawada Masahiko
Discussion: e9f33d68f1042288859e78d5d58add3038dc59d9@G08CNEXMBPEKD06.g08.fujitsu.local
Backpatch-through: 9.6
(cherry picked from commit 8f4ee44bcdbcd437f860f2d048125a118335b088)

lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb