Bluetooth: Handle get RSSI timeout gracefully

Usecase:
1. Turn ON BT
2. Pair and connect with Headset
3. Play Music over A2DP
4. Make GetRssi timeout
5. Bt app crashes

Failure:
BT app crashes.

Rootcause:
Calling callback with NULL and in callback handling NULL check with assert.

Fix:
Call the callback with error instead of NULL.

Change-Id: I20a9e504a513697945cd76d218ec12a0ef1e3330