pg_dump: Don't leak memory in buildDefaultACLCommands()

buildDefaultACLCommands() didn't destroy the string buffer created in
certain cases, leading to a memory leak.  Fix by destroying the buffer
before returning from the function.

Spotted by Coverity.

Author: Michael Paquier

Back-patch to 9.6 where buildDefaultACLCommands() was added.

src/bin/pg_dump/dumputils.c

@@ -394,13 +394,19 @@ buildDefaultACLCommands(const char *type, const char *nspname,
 		appendPQExpBuffer(sql, "SELECT pg_catalog.binary_upgrade_set_record_init_privs(true);\n");
 		if (!buildACLCommands("", NULL, type, initacls, initracls, owner,
 							  prefix->data, remoteVersion, sql))
+		{
+			destroyPQExpBuffer(prefix);
 			return false;
+		}
 		appendPQExpBuffer(sql, "SELECT pg_catalog.binary_upgrade_set_record_init_privs(false);\n");
 	}
 
 	if (!buildACLCommands("", NULL, type, acls, racls, owner,
 						  prefix->data, remoteVersion, sql))
+	{
+		destroyPQExpBuffer(prefix);
 		return false;
+	}
 
 	destroyPQExpBuffer(prefix);