hub.py: do sys.exc_clear() after switch_out() rather than before

gevent/hub.py

@@ -171,13 +171,13 @@ class Hub(greenlet):
         assert cur is not self, 'Impossible to call blocking function in the event loop callback'
         exc_info = sys.exc_info()
         try:
-            sys.exc_clear()
             switch_out = getattr(cur, 'switch_out', None)
             if switch_out is not None:
                 try:
                     switch_out()
                 except:
                     traceback.print_exc()
+            sys.exc_clear()
             return greenlet.switch(self)
         finally:
             core.set_exc_info(*exc_info)