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 e2090d9d, so backpatch down to 9.6.

Author: Jie Zhang
Reviewed-by: Sawada Masahiko
Discussion: https://postgr.es/m/05bcbc5857f948efa0b451b85a48ae10@G08CNEXMBPEKD06.g08.fujitsu.local
Backpatch-through: 9.6

src/bin/pg_dump/pg_dump.c

@@ -9592,6 +9592,11 @@ getDefaultACLs(Archive *fout, int *numDefaultACLs)
 						  racl_subquery->data,
 						  initacl_subquery->data,
 						  initracl_subquery->data);
+
+		destroyPQExpBuffer(acl_subquery);
+		destroyPQExpBuffer(racl_subquery);
+		destroyPQExpBuffer(initacl_subquery);
+		destroyPQExpBuffer(initracl_subquery);
 	}
 	else
 	{