VolumeControlService: Fix for possible crash

Could happen when profile is stopped while connecting.

Crash Messages sorted from most recent:
Attempt to invoke virtual method 'int com.android.bluetooth.btservice.AdapterService.getBondState(android.bluetooth.BluetoothDevice)' on a null object reference
java.lang.NullPointerException: Attempt to invoke virtual method 'int com.android.bluetooth.btservice.AdapterService.getBondState(android.bluetooth.BluetoothDevice)' on a null object reference
	at com.android.bluetooth.vc.VolumeControlService.okToConnect(VolumeControlService.java:293)
	at com.android.bluetooth.vc.VolumeControlStateMachine$Disconnected.processMessage(VolumeControlStateMachine.java:140

Bug: 206116821
Test: atest VolumeControlServiceTest
Change-Id: I0b8580e1f8e8e0b8e507cd1147b227880b4ee1d5