Merge branch 'sn/null-pointer-arith-in-mark-tree-uninteresting'

mark_tree_uninteresting() has code to handle the case where it gets
passed a NULL pointer in its 'tree' parameter, but the function had
'object = &tree->object' assignment before checking if tree is
NULL.  This gives a compiler an excuse to declare that tree will
never be NULL and apply a wrong optimization.  Avoid it.

* sn/null-pointer-arith-in-mark-tree-uninteresting:
  revision.c: fix possible null pointer arithmetic

revision.c

@@ -135,10 +135,12 @@ 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;