[media] pvrusb2: check for allocation failures

This function returns NULL on failure so lets do that if kzalloc()
fails.  There is a separate problem that the caller for this function
doesn't check for errors...

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-By: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>