OPP: Fix NPE issue while turn off BT

Use Case:
1. Pair remote with DUT.
2. From gallery send a file to remote
3. Turn off BT
4. Check the file transfer should fail

Failure:
BT crash seen when we turn off BT during file transfer.

Root Cause:
In Some conditions Bt Socket instance is remains null
because of turn off BT.

Fix:
Before closing connection check BT Socket instance
Value to avoid NPE.

Change-Id: Idcd90c31a179247055a155678d11d8e76ec90aca