Fix small query-lifespan memory leak in bulk updates.

When there is an identifiable REPLICA IDENTITY index on the target table,
heap_update leaks the id_attrs bitmapset.  That's not many bytes, but it
adds up over enough rows, since the code typically runs in a query-lifespan
context.  Bug introduced in commit e55704d8b, which did a rather poor job
of cloning the existing use-pattern for RelationGetIndexAttrBitmap().

Per bug #14293 from Zhou Digoal.  Back-patch to 9.4 where the bug was
introduced.

Report: <20160824114320.15676.45171@wrigleys.postgresql.org>

src/backend/access/heap/heapam.c

@@ -3467,6 +3467,7 @@ l2:
 			ReleaseBuffer(vmbuffer);
 		bms_free(hot_attrs);
 		bms_free(key_attrs);
+		bms_free(id_attrs);
 		return result;
 	}
 
@@ -3928,6 +3929,7 @@ l2:
 
 	bms_free(hot_attrs);
 	bms_free(key_attrs);
+	bms_free(id_attrs);
 
 	return HeapTupleMayBeUpdated;
 }