Changes

lchan_fsm: ignore late lchan_rtp_fsm events

The lchan_rtp_fsm is a child FSM that manages the MGW/RTP endpoint.
When the parent lchan_fsm transitions away before the child has
finished its work (e.g. a CRCX timeout or a DLCX completing after
the lchan is already idle), the child can still deliver
LCHAN_EV_RTP_RELEASED or LCHAN_EV_RTP_ERROR to the parent.

Currently these late events are not in the in_event_mask of every
state that can be reached with the child still running:

* LCHAN_ST_UNUSED: entered from WAIT_AFTER_ERROR after the error timer
  fires, while a DLCX triggered at error time may still be in flight.
* LCHAN_ST_WAIT_AFTER_ERROR: already handles LCHAN_EV_RTP_RELEASED but
  misses LCHAN_EV_RTP_ERROR (e.g. CRCX timeout arriving after the
  NACK was handled).

Add the missing events to both states' in_event_mask and provide a
no-op handler in lchan_fsm_unused() so that the assert is not hit.

Change-Id: Ie6333bd941e4e5a6ddf0e3f113b8764e8bc2bbc0