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

--HG--
extra : transplant_source : %ED%01_%E4-Z%FC5%0C%89%AA%E8%B9%8A%FD%28%C47%81L

gevent/hub.py

@@ -153,13 +153,13 @@ class Hub(greenlet):
         assert cur is not self, 'Cannot switch to MAINLOOP from MAINLOOP'
         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)