dbus: Fix Scan invalid args reporting (uninitialize variable)

Must initialize key to be NULL since it can be used in an error
message. If the Scan method did not actually include any dict entries,
the uninitialized pointer was used and this could result in
wpa_supplicant crashing.

wpa_supplicant/dbus/dbus_new_handlers.c

@@ -905,7 +905,7 @@ DBusMessage * wpas_dbus_handler_scan(DBusMessage *message,
 	DBusMessage * reply = NULL;
 	DBusMessageIter iter, dict_iter, entry_iter, variant_iter,
 		array_iter, sub_array_iter;
-	char *key, *val, *type = NULL;
+	char *key = NULL, *val, *type = NULL;
 	int len;
 	int freqs_num = 0;
 	int ssids_num = 0;