Enable specifying no events when registring with the iomux

src/iomux_select.c

@@ -65,7 +65,7 @@ static int iomux_select_register_fd(struct IOMux *mux_, struct IOMuxFD *fd)
     struct IOMuxSelect *mux = mux_->instance;
     int rc;
 
-    if ((fd->what & MUX_EV_MASK) == 0 || fd->callback == NULL)
+    if (fd->callback == NULL)
         return EINVAL;
 
     rc = iomux_select_update_events(mux_, fd);