tools: ynl: error check scanf() in a sample

Someone reported on GitHub that the YNL NIPA test is failing
when run locally. The test builds the tools, and it hits:

  netdev.c:82:9: warning: ignoring return value of ‘scanf’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  82 | scanf("%d", &ifindex);

I can't repro this on my setups but error seems clear enough.

Link: https://github.com/linux-netdev/nipa/discussions/37Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://patch.msgid.link/20240828173609.2951335-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/samples/netdev.c

@@ -79,7 +79,10 @@ int main(int argc, char **argv)
 		goto err_close;
 
 	printf("Select ifc ($ifindex; or 0 = dump; or -2 ntf check): ");
-	scanf("%d", &ifindex);
+	if (scanf("%d", &ifindex) != 1) {
+		fprintf(stderr, "Error: unable to parse input\n");
+		goto err_destroy;
+	}
 
 	if (ifindex > 0) {
 		struct netdev_dev_get_req *req;
@@ -119,6 +122,7 @@ int main(int argc, char **argv)
 
 err_close:
 	fprintf(stderr, "YNL: %s\n", ys->err.msg);
+err_destroy:
 	ynl_sock_destroy(ys);
 	return 2;
 }