Add null checks for L2CAP socket callback

Add null checks for L2CAP socket callback

Use Case: Bluetooth process crashed while sending the file
to remote device.

Steps: Send a file over L2CAP (OBEX over L2CAP) to remote device

Failure: BT process will crash and restarted automatically

Root Cause: L2CAP socket callback reset to null on error
condition, and when other function try to dereference it, this
leads to BT crash.

Fix: Added null checks for L2CAP socket callback

Change-Id: I2e4f20278fcc8a09bd4dbd507a6c4147e0de93c1