Fix resource leaks in pool_memqcache.c.

These leaks were brought in by commit 6fdba5c " Use psprintf()
instead of snprintf()."  Since the commit was backpatched through 4.1,
this needs to be backpatched through 4.1 too.

Per Coverity (CID 1559726).
Backpatch-through: 4.1.

src/query_cache/pool_memqcache.c

@@ -1743,6 +1743,7 @@ pool_add_table_oid_map(POOL_CACHEKEY * cachekey, int num_table_oids, int *table_
 					(errmsg("memcache: adding table oid maps, failed to write file:\"%s\"", path),
 					 errdetail("%m")));
 			close(fd);
+			pfree(path);
 			return;
 		}
 		close(fd);