[media] v4l2-dv-timings: add sanity checks in cvt,gtf calculations

Wrong values of hfreq and image height can lead to strange timings.
Avoid timing calculations for such values.

Suggested By: Martin Bugge <marbugge@cisco.com>

Cc: Martin Bugge <marbugge@cisco.com>
Signed-off-by: Prashant Laddha <prladdha@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>