attr: fix leak in free_attr_elem

This function frees the individual "struct match_attr"s we
have allocated, but forgot to free the array holding their
pointers, leading to a minor memory leak (but it can add up
after checking attributes for paths in many directories).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -286,6 +286,7 @@ static void free_attr_elem(struct attr_stack *e)
 		}
 		free(a);
 	}
+	free(e->attrs);
 	free(e);
 }