Added current_block = NULL to quickStop()

Marlin/stepper.cpp

@@ -822,6 +822,7 @@ void quickStop()
   DISABLE_STEPPER_DRIVER_INTERRUPT();
   while(blocks_queued())
     plan_discard_current_block();
+  current_block = NULL;
   ENABLE_STEPPER_DRIVER_INTERRUPT();
 }