AsyncMessenger: reset newly allocated file_events

* otherwise there is chance that Event could take them as already
  initialized ones even they are filled with random bits from heap.

src/msg/async/Event.cc

@@ -120,6 +120,7 @@ int EventCenter::create_file_event(int fd, int mask, EventCallbackRef ctxt)
       lderr(cct) << __func__ << " failed to realloc file_events" << cpp_strerror(errno) << dendl;
       return -errno;
     }
+    memset(file_events+sizeof(FileEvent)*nevent, 0, sizeof(FileEvent)*(new_size-nevent));
     file_events = new_events;
     nevent = new_size;
   }