tree: move assignments of ->tree_errno up a bit

Both tree_ascend() and tree_pop() may cause errno to be
set/clobbered. As for the latter, we're certainly not interested in
the random values that free() etc. may leave in errno.

If tree_ascend() fails, its return value takes precedence in
determining t->visit_type and the return value, but tree_ascend() also
updates t->tree_errno appropriately, which makes sense.

But when tree_ascend() succeeds, t->tree_errno should reflect the
reason for opendir() failing, which requires that we record errno
immediately.

Bug bounty:	$20

tar/tree.c

@@ -438,16 +438,16 @@ tree_next(struct tree *t)
 			t->dirname_length = t->path_length;
 			if (chdir(t->stack->name) != 0) {
 				/* chdir() failed; return error */
-				tree_pop(t);
 				t->tree_errno = errno;
+				tree_pop(t);
 				return (t->visit_type = TREE_ERROR_DIR);
 			}
 			t->depth++;
 			t->d = opendir(".");
 			if (t->d == NULL) {
+				t->tree_errno = errno;
 				r = tree_ascend(t); /* Undo "chdir" */
 				tree_pop(t);
-				t->tree_errno = errno;
 				t->visit_type = r != 0 ? r : TREE_ERROR_DIR;
 				return (t->visit_type);
 			}