[media] Guard a divide in v4l1 compat layer

Hi,
  I managed to trigger a divide by 0 in the v4l compat code
with the mem2mem test module; I suspect perhaps it shouldn't
have been returning a 0 pixel wide picture, but either way it seems
right to guard this divide by 0 in the compatibility layer.

Tested on 2.6.36 (ubuntu build, but the code in this is the same as upstream),
but ***not tested with a real video device***.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
cc: stable.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>