revision.c: fix possible null pointer arithmetic

mark_tree_uninteresting() dereferences a tree pointer before
checking if the pointer is valid. Fix that by doing the check first.
Signed-off-by: NStefan Naewe <stefan.naewe@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

revision.c

@@ -131,10 +131,12 @@ static void mark_tree_contents_uninteresting(struct tree *tree)
 
 void mark_tree_uninteresting(struct tree *tree)
 {
-	struct object *obj = &tree->object;
+	struct object *obj;
 
 	if (!tree)
 		return;
+
+	obj = &tree->object;
 	if (obj->flags & UNINTERESTING)
 		return;
 	obj->flags |= UNINTERESTING;