V4L/DVB (13875): gspca - vc032x: Fix a possible crash with the vc0321 bridge.

The frame pointer returned by get_i_frame may be NULL when the application
is too slow.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>