Return 0 from PollIoBackend::eb_event_add if the event type is not set

Summary: Return 0 from PollIoBackend::eb_event_add if the event type is not set

Reviewed By: danobi

Differential Revision: D21416811

fbshipit-source-id: 5825e473dca06d33017785e4d73947aade5f554c

folly/experimental/io/PollIoBackend.cpp

@@ -578,7 +578,7 @@ int PollIoBackend::eb_event_add(Event& event, const struct timeval* timeout) {
     }
   }
 
-  return -1;
+  return 0;
 }
 
 int PollIoBackend::eb_event_del(Event& event) {