sgsn: Cleanup after RA Update Reject / Attach Reject (TODO)

Currently, the LLME is not cleanup up after sending an RA Update
Reject. This happens after entering a routing area from outside,
since in that case the SGSN sends an RA Update Reject (implicitly
detached) which causes the MS to restart the attach procedure.
The LLME is also not updated if an Attach Request with message errors
(encoding, invalid MI type) is received or if an MM context cannot be
allocated.

This patch changes gsm48_rx_gmm_ra_upd_req and gsm48_rx_gmm_att_req
to unassign the LLME or free the MM context (if available) after a
Reject message has been sent.

TODO:
 - test cases

Ticket: OW#1324
Sponsored-by: On-Waves ehf
debian: Prepare new test release (0.13.1+z8f)