GAS: Fix OSU Providers List response with invalid configuration

If the OSU Providers configuration was invalid (included osu_server_uri
but not osu_method_list), it was possible for the GAS response
generation to hit a NULL dereference. Fix this by checking for the
invalid configuration before trying to fill in the OSU methods.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>

src/ap/gas_serv.c

@@ -691,7 +691,7 @@ static void anqp_add_osu_provider(struct wpabuf *buf,
 
 	/* OSU Method List */
 	count = wpabuf_put(buf, 1);
-	for (i = 0; p->method_list[i] >= 0; i++)
+	for (i = 0; p->method_list && p->method_list[i] >= 0; i++)
 		wpabuf_put_u8(buf, p->method_list[i]);
 	*count = i;