lib: tweak selection errors when consuming cached answer

Forwarding-related changes uncovered this case
in integration.iter_limits test.  Errors can happen also
when consuming data produced by cache, in which case there's
no suitable server to blame for the error.

lib/resolve-produce.c

@@ -646,6 +646,7 @@ int kr_resolve_produce(struct kr_request *request, struct kr_transport **transpo
 		ITERATE_LAYERS(request, qry, produce, packet);
 		if (!(request->state & KR_STATE_FAIL) && knot_wire_get_qr(packet->wire)) {
 			/* Produced an answer from cache, consume it. */
+			kr_server_selection_cached(qry);
 			qry->secret = 0;
 			request->state = KR_STATE_CONSUME;
 			ITERATE_LAYERS(request, qry, consume, packet);

lib/selection.c

@@ -772,6 +772,15 @@ void kr_server_selection_init(struct kr_query *qry)
 			mempool, &qry->server_selection.local_state->private);
 	}
 }
+void kr_server_selection_cached(struct kr_query *qry)
+{
+	qry->server_selection = (struct kr_server_selection){
+		.initialized = false,
+		// we reuse iter_error, as it's no-op if (!initialized)
+		.error = iter_error,
+		// everything else is NULL
+	};
+}
 
 int kr_forward_add_target(struct kr_request *req, const struct sockaddr *sock)
 {

lib/selection.h

@@ -131,6 +131,12 @@ struct kr_server_selection {
  */
 KR_EXPORT
 void kr_server_selection_init(struct kr_query *qry);
+/**
+ * @brief Ensure server selection state suitable for processing "reply from cache".
+ *
+ * In particular, qry->server_selection.error() calls shouldn't crash.
+ */
+void kr_server_selection_cached(struct kr_query *qry);
 
 /**
  * @brief Add forwarding target to request.