V4L/DVB (13016): kmalloc failure ignored in lgdt3304_attach() and s921_attach()

Prevent NULL dereference if kmalloc() fails.

Cc: Markus Rechberger <mrechberger@sundtek.de>
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>