BTGatt: Fix potential NPE during monkey test.

Use case:
During Monkey test

Root cause:
Race condition between Gat Service and binder call from
system server.

Fix:
Add null check before calling stopscan and stopadvertising sets
to prevent NPE errors during random test.

CRs-Fixed: 2250000
Change-Id: I5c01d3eade4a6a4a455ac4a381152e0058f9a727
1 file changed