VLAN: Check vlan_desc validity in a failure debug print

The recent VLAN changes added an explicit code path that sets vlan_desc
= NULL within ap_sta_set_vlan(). This makes some code analyzers warn
about the debug print that could potentially dereference this pointer.
Silence that warning by verifying the pointer more consistently within
this function.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/ap/sta_info.c

@@ -900,8 +900,9 @@ int ap_sta_set_vlan(struct hostapd_data *hapd, struct sta_info *sta,
 				       HOSTAPD_MODULE_IEEE80211,
 				       HOSTAPD_LEVEL_DEBUG,
 				       "could not add dynamic VLAN interface for vlan=%d%s",
-				       vlan_desc->untagged,
-				       vlan_desc->tagged[0] ? "+" : "");
+				       vlan_desc ? vlan_desc->untagged : -1,
+				       (vlan_desc && vlan_desc->tagged[0]) ?
+				       "+" : "");
 			vlan_id = 0;
 			ret = -1;
 			goto done;