GATT: Ensure JNI callback object is valid before callback invocation.

Use case:
Perform BT ON/OFF in stress test.

Issue:
Bluetooth process cra_shes.

Root Cause:
When BT is turned OFF,  JNI Callback object was set to NULL in
cleanupNative.

Fix:
Guard callback invocation with NULL check.

CRs-Fixed: 2447396
Change-Id: If931f0d95fa97426947f2eccb26c4cec48696b65
1 file changed