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

net/ipv4/cipso_ipv4.c

@@ -476,7 +476,7 @@ int cipso_v4_doi_add(struct cipso_v4_doi *doi_def,
 	doi = doi_def->doi;
 	doi_type = doi_def->type;
 
-	if (doi_def == NULL || doi_def->doi == CIPSO_V4_DOI_UNKNOWN)
+	if (doi_def->doi == CIPSO_V4_DOI_UNKNOWN)
 		goto doi_add_return;
 	for (iter = 0; iter < CIPSO_V4_TAG_MAXCNT; iter++) {
 		switch (doi_def->tags[iter]) {