[media] dib8000: move dereference after check for NULL

My static checker complains that we dereference "state" inside the call
to fft_to_mode() before checking for NULL.  The comments say that it is
possible for "state" to be NULL so I have moved the dereference after
the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>