Add msg->len sanity checks to fix static checker warning:

	include/linux/cec-funcs.h:1154 cec_ops_set_osd_string()
	warn: setting length 'msg->len - 3' to negative one
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>