MountService: Check for null pointer when looking for vold code

* There is actually no code path that will set an event into
  NativeDaemonConnectorException, so any reference to the event code
  will result in a null pointer exception, taking down MountService and
  the rest of the system with it.

Change-Id: Id62f724953aacc363b3455584c6a3ad1381c6138