[media] cec: add sanity check for msg->len

Check (and warn) if the msg->len is too long or if it is 0.

Should never happen, but just in case...

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>