[media] gspca/stv06xx_pb0100: Don't potentially deref NULL in pb0100_start()

usb_altnum_to_altsetting() may return NULL. If it does we'll dereference a
NULL pointer in
drivers/media/video/gspca/stv06xx/stv06xx_pb0100.c::pb0100_start().
As far as I can tell there's not really anything more sensible than
-ENODEV that we can return in that situation, but I'm not at all intimate
with this code so I'd like a bit of review/comments on this before it's
applied.
Anyway, here's a proposed patch.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Cc: Jean-Francois Moine <moinejf@free.fr>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Erik Andren <erik.andren@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>