V4L/DVB (13017): gspca: kmalloc failure ignored in sd_start()

Prevent NULL dereference if kmalloc() fails.

Cc: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>