RFCOMM: Crash on null MCB and PORT in state machine

* rfc_mx_sm_execute should never receive a NULL mcb
* rfc_port_sm_execute should never receive a NULL port
* If the above happens, we should crash since otherwise we might miss a
  state machine event and not knowing that we missed it if we just do a
  simple NULL check

Bug: 77224743
Test: StackRfcommTest
Change-Id: I7114e46ae706927cc839c7201c97362710e7a874