Free object allocated at quorum_register_callback

Memory object allocated with malloc at quorum_register_callback
is not freed. The object is linked to internal_trackers_list.

The object is unlinked at quorum_unregister_callback. However,
it is not freed at the function.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>