[media] gspca - main: Check the isoc packet status before its length

When an error is set for an isochronous packet, the length of the packet
may be null. In this case, the error was not detected and the image
was not discarded as it should be.

Reported-by: Franck Bourdonnec <fbourdonnec@chez.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>