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: https://postgr.es/m/
05bcbc5857f948efa0b451b85a48ae10@G08CNEXMBPEKD06.g08.fujitsu.local
Backpatch-through: 9.6