sbus: free watch_fd->fdevent explicitly

We never reproduced this with gdb but valgrind shows invalid read in sbus_watch_handler
after the watch_fd was freed. This should not be needed since watch_fd is memory parent
of fdevent but it seems to help.

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

src/sbus/connection/sbus_watch.c

@@ -280,6 +280,7 @@ sbus_watch_remove(DBusWatch *dbus_watch, void *data)
 
     if (watch_fd->dbus_watch.read == NULL
             && watch_fd->dbus_watch.write == NULL) {
+        talloc_free(watch_fd->fdevent);
         talloc_free(watch_fd);
     }
 }