idle_timeout: exit the timer if the printer is shutdown

This resolves an issue where the timer can get stuck
in an attempt to transition from Ready to Idle if
the printer is shutdown.

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

klippy/extras/idle_timeout.py

@@ -71,6 +71,8 @@ class IdleTimeout:
         # Idle timeout has elapsed
         return self.transition_idle_state(eventtime)
     def timeout_handler(self, eventtime):
+        if self.printer.is_shutdown():
+            return self.reactor.NEVER
         if self.state == "Ready":
             return self.check_idle_timeout(eventtime)
         # Check if need to transition to "ready" state