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)

0 files changed, 0 insertions, 0 deletions