cipso: remove an unneeded NULL check in cipso_v4_doi_add()

We dereference doi_def on the line before the NULL check.  It has
been this way since 2008.  I checked all the callers and doi_def is
always non-NULL here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>