Bluetooth: Fix to avoid PAN JNI crash due to ENABLE_TIMEOUT

A case, where BT ON/OFF is running in the loop(stress). In one
of the iteration, BT ON gets failed due to ENABLE_TIMEOUT.
But BTIF for PAN had already been initialized. SSR cleanup is
triggered due to BT on failure. So PAN JNI callback obj gets deleted,
PAN control_state_callback is triggered just after PAN JNI cleanup.
BT gets crashed. Checking for NULL to avoid crash in any unknown
scenario.

CRs-fixed: 919741
Change-Id: Ie95470348f11be6c9eccc1e4ac3869d6789f3f6c