Fix crash when reading the empty tree

cvsimport needs to call git-read-tree without arguments to create an empty
tree.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

read-tree.c

@@ -881,7 +881,7 @@ int main(int argc, char **argv)
 	 * valid cache-tree because the index must match exactly
 	 * what came from the tree.
 	 */
-	if (trees->item && (!merge || (stage == 2))) {
+	if (trees && trees->item && (!merge || (stage == 2))) {
 		cache_tree_free(&active_cache_tree);
 		prime_cache_tree();
 	}