unpack_trees: plug leakage of o->result

Most of the time the caller specifies to which destination variable
the resulting index_state should be assigned by passing a non-NULL
pointer in o->dst_index to receive that state, but for a caller that
gives a NULL o->dst_index, the resulting index simply leaked.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -1157,6 +1157,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	if (o->dst_index) {
 		discard_index(o->dst_index);
 		*o->dst_index = o->result;
+	} else {
+		discard_index(&o->result);
 	}
 
 done: