lapd: Check in rslms_rx_rll() if lapdm context was initialized earlier

This was found while implementing handover on a sysmobts. When we
receive a channel release request for a channel that was never really
activated (set_lapdm_context() was not called) we segfault in
lapd_recv_dlsap().

We now return early with -EINVAL in rslms_rx_rll() if we receive a
message that assumes set_lapdm_context() was already called.

These are:
* RSL_MT_UNIT_DATA_REQ
* RSL_MT_DATA_REQ
* RSL_MT_SUSP_REQ
* RSL_MT_REL_REQ

A test case was added to trigger the issue.
3 files changed