Callbacks were set even attach failed. This leads calling
.release() in error case and resulted crash.
Reported-by: NOliver Schinagl <oliver@schinagl.nl>
Signed-off-by: NAntti Palosaari <crope@iki.fi>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>