kill: check if greenlet is alive before doing anything

gevent/greenlet.py

@@ -101,6 +101,7 @@ def kill(greenlet, exception=GreenletExit, block=False, polling_period=0.2):
     """Kill greenlet with exception (GreenletExit by default).
     Wait for it to die if block is true.
     """
+    if not greenlet.dead:
         waiter = Waiter()
         core.active_event(_kill, greenlet, exception, waiter)
         if block: