media: cedrus: h265: Add a couple of error checks

Now that we have infrastructure for reporting errors, let's add two
checks, which will make sure slice can be actually decoded.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>