found bug causing security check failure. routing now works in more cases.

Test cases run (using routeleaftest):
1. Tree = Coord(0x0000)-Router(0x2000)-Leaf(0x2001); traffic 0x2001<->0x0000. Works.
2. Tree = Leaf(0x0001)-Coord(0x0000)-Router(0x2000); traffic 0x2000<->0x0001. Works.

TODO:
* Fix assertion failure.
* Deal with TODO regarding MLME-COMM-STATUS.indication

Test cases to run:
* Tree = Leaf1-Coord-Router-Leaf2; traffic Leaf1<->Leaf2.
* Tree = Leaf1-Coord-Router-Leaf2; traffic Leaf1<->Router.
* Tree = Coord-Router[1-3]; traffic RouterX<->RouterY