Stop accessing statemachine after cleanup

Usecase:
-pair and connect DUT with Remote
-Start A2DP streaming
-Initiate disconnection from DUT

Failure:
Bluetoth crashed after initiating disconnect from DUT

Rootcause:
Accessing state machine context after cleanup.

Fix:
Bluetooth: Added null check to stop accessing state machine after cleanup

Change-Id: I85ca55aaa0b7604125e45a62e32ae02eca605e43