Properly use return value of select to count the number of events

src/iomux_epoll.c

@@ -183,7 +183,7 @@ again:
             fd->callback(mux_, fd, events);
             //be sure not to use fd after the callback, it might been removed.
 
-            mux->pending_events[i].data.ptr = NULL;
+            mux->pending_events[i].data.ptr = NULL; //clear from pending list
 
             event_cnt++;
         }