auth: Access always first entry when flushing failures

The code is deleting the first item after accessing
it, and then moving forward. It will eventually go
beyond the array and get NULL ptr and fail.

Instead we need to always get the first item,
since the array deletion is moving the queued items
forward.

Broken by e18b4e4

src/auth/auth-request-handler.c

@@ -843,9 +843,10 @@ void auth_request_handler_flush_failures(bool flush_all)
 
 	/* flush the requests */
 	for (i = 0; i < count; i++) {
-		auth_request = auth_requests[aqueue_idx(auth_failures, i)];
+		auth_request = auth_requests[aqueue_idx(auth_failures, 0)];
 		aqueue_delete_tail(auth_failures);
 
+		i_assert(auth_request != NULL);
 		i_assert(auth_request->state == AUTH_REQUEST_STATE_FINISHED);
 		auth_request_handler_reply(auth_request,
 					   AUTH_CLIENT_RESULT_FAILURE,