message: PV: shut down threads only if initialized
  Fixes
diff: @@ -165,8 +165,11 @@ static boost::scoped_ptr<ECCVerifyHandle> globalVerifyHandle;
  void Interrupt(boost::thread_group& threadGroup)
  {
  // Interrupt Parallel Block Validation threads if there are any running.
  + if (PV)
  + {
  PV->StopAllValidationThreads();
  PV->WaitForAllValidationThreadsToStop();
  + }
  InterruptHTTPServer();
  InterruptHTTPRPC();
  