mq_tree_foreach_from(): Fix upward traversal

Steps to reproduce bug:

 1. From tab 1, append a new child
 2. From tab 1, insert a new sibling
 3. From tab 1, insert a new sibling

The position of the last tab (the next sibling of the new tab inserted
in step 3) gets updated twice.

Tree debugging output:

    Traversing tree from position, starting at 0x12bf620
            0x12bf620
            0x1272f80
            0x1272f80
    Inserted sibling 0x12bf620, new tree (size 5):
            0xe7f7f0 (size 5, position 0)
                    0xed3e10 (size 2, position 1)
                            0x114ef40 (size 1, position 2)
                    0x12bf620 (size 1, position 3)
                    0x1272f80 (size 1, position 5)

1 files changed, 9 insertions, 8 deletions

diff --git a/src/tree.c b/src/tree.c
index ece8594..c3e2323 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -254,16 +254,17 @@ static gboolean
 foreach_up(MqTree *node, gboolean (*cb)(MqTree *node, gpointer user_data),
 	gpointer user_data)
 {
-	if (node->next) {
-		if (foreach_down(node->next, cb, user_data) == MQ_TREE_STOP) {
-			return MQ_TREE_STOP;
-		}
+	node = node->parent;
+	if (!node) {
+		return MQ_TREE_CONTINUE;
 	}
 
-	if (node->parent) {
-		if (foreach_up(node->parent, cb, user_data) == MQ_TREE_STOP) {
-			return MQ_TREE_STOP;
-		}
+	if (foreach_down(node->next, cb, user_data) == MQ_TREE_STOP) {
+		return MQ_TREE_STOP;
+	}
+
+	if (foreach_up(node, cb, user_data) == MQ_TREE_STOP) {
+		return MQ_TREE_STOP;
 	}
 
 	return MQ_TREE_CONTINUE;