Bug 993954 - Check sBluetoothChild is null when closing Settings application

It is possible that sBluetoothChild set to nullptr and RecvNotify get called from MessageLoop.
This patch check sBluetoothChild.

dom/bluetooth/ipc/BluetoothChild.cpp

@@ -75,7 +75,9 @@ BluetoothChild::RecvNotify(const BluetoothSignal& aSignal)
 {
   MOZ_ASSERT(sBluetoothService);
 
+  if (sBluetoothService) {
     sBluetoothService->DistributeSignal(aSignal);
+  }
   return true;
 }
 
@@ -84,7 +86,9 @@ BluetoothChild::RecvEnabled(const bool& aEnabled)
 {
   MOZ_ASSERT(sBluetoothService);
 
+  if (sBluetoothService) {
     sBluetoothService->SetEnabled(aEnabled);
+  }
   return true;
 }