m-reserve-device: use a weak ref instead of raw pointer for transition

This always guarantees the transition is valid when g_weak_ref_get() does not
return NULL.
2 jobs for coverity-transition-test-fix in 1 minute and 12 seconds (queued for 1 second)
Status Name Job ID Coverage
  Container
passed container_fedora #11294182

00:00:21

 
  Build
passed build_on_fedora #11294183

00:00:50