core.pyx: add an assert to event's callback handler

gevent/core.pyx

@@ -108,6 +108,7 @@ EV_PERSIST = 0x10
 cdef void __event_handler(int fd, short evtype, void *arg) with gil:
     cdef event ev = <event>arg
     try:
+        assert ev.fd == fd, (ev.fd, fd)
         ev._callback(ev, evtype)
     except:
         traceback.print_exc()