mac80211: fix debugfs default key oops

Under certain circumstances (in AP mode) the debugfs function
that is supposed to add the default key symlink can encounter
a NULL default_key pointer. This patch makes it handle that
situtation gracefully.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/debugfs_key.c

@@ -255,14 +255,23 @@ void ieee80211_debugfs_key_remove(struct ieee80211_key *key)
 void ieee80211_debugfs_key_add_default(struct ieee80211_sub_if_data *sdata)
 {
 	char buf[50];
+	struct ieee80211_key *key;
 
 	if (!sdata->debugfsdir)
 		return;
 
-	sprintf(buf, "../keys/%d", sdata->default_key->debugfs.cnt);
-	sdata->debugfs.default_key =
-		debugfs_create_symlink("default_key", sdata->debugfsdir, buf);
+	/* this is running under the key lock */
+
+	key = sdata->default_key;
+	if (key) {
+		sprintf(buf, "../keys/%d", key->debugfs.cnt);
+		sdata->debugfs.default_key =
+			debugfs_create_symlink("default_key",
+					       sdata->debugfsdir, buf);
+	} else
+		ieee80211_debugfs_key_remove_default(sdata);
 }
+
 void ieee80211_debugfs_key_remove_default(struct ieee80211_sub_if_data *sdata)
 {
 	if (!sdata)