DPP: Clear hapd->gas pointer on deinit

While it does not look like the stale pointer could have been
dereferenced in practice, it is better not to leave the stale pointer to
freed memory in place to avoid accidental uses.

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

src/ap/hostapd.c

@@ -392,6 +392,7 @@ void hostapd_free_hapd_data(struct hostapd_data *hapd)
 #ifdef CONFIG_DPP
 	hostapd_dpp_deinit(hapd);
 	gas_query_ap_deinit(hapd->gas);
+	hapd->gas = NULL;
 #endif /* CONFIG_DPP */
 
 	authsrv_deinit(hapd);