Plug memory leak in index_get_partition

The list of indexes was being leaked when asked for an index that
doesn't have an index partition in the table partition.  Not a common
case admittedly --and in most cases where it occurs, caller throws an
error anyway-- but worth fixing for cleanliness and in case any
third-party code is calling this function.

While at it, remove use of lfirst_oid() to obtain a value we already
have.

Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20201105203606.GF22691@telsasoft.com

src/backend/catalog/partition.c

@@ -170,13 +170,14 @@ index_get_partition(Relation partition, Oid indexId)
 		ReleaseSysCache(tup);
 		if (!ispartition)
 			continue;
-		if (get_partition_parent(lfirst_oid(l)) == indexId)
+		if (get_partition_parent(partIdx) == indexId)
 		{
 			list_free(idxlist);
 			return partIdx;
 		}
 	}
 
+	list_free(idxlist);
 	return InvalidOid;
 }