wifi: return error for AddAndActivate requests for hidden APs

If there's no SSID, we can't connect at all.  So if a client passes
in a hidden AP, and doesn't send the SSID in the partial connection
info, we can't make a connection with it.  Return an error instead
of crashing.

src/nm-device-wifi.c

@@ -1460,6 +1460,26 @@ real_complete_connection (NMDevice *device,
 	if (ap) {
 		ssid = nm_ap_get_ssid (ap);
 
+		if (ssid == NULL) {
+			/* The AP must be hidden.  Connecting to a WiFi AP requires the SSID
+			 * as part of the initial handshake, so check the connection details
+			 * for the SSID.  The AP object will still be used for encryption
+			 * settings and such.
+			 */
+			ssid = nm_setting_wireless_get_ssid (s_wifi);
+		}
+
+		if (ssid == NULL) {
+			/* If there's no SSID on the AP itself, and no SSID in the
+			 * connection data, then we cannot connect at all.  Return an error.
+			 */
+			g_set_error_literal (error,
+			                     NM_WIFI_ERROR,
+			                     NM_WIFI_ERROR_CONNECTION_INVALID,
+			                     "A 'wireless' setting with a valid SSID is required for hidden access points.");
+			return FALSE;
+		}
+
 		/* If the SSID is a well-known SSID, lock the connection to the AP's
 		 * specific BSSID so NM doesn't autoconnect to some random wifi net.
 		 */