print-editor: Fix a wrong comparison

If error is NULL, replacing the file contents worked and we don't
show the error dialog. The previous version pretty explicitly
dereferenced a NULL pointer by first ensuring that error is NULL and
then accessing error->message.

Found by scan-build

demos/print-editor/print-editor.c

@@ -164,7 +164,7 @@ save_file (GFile *save_filename)
                            NULL,
                            &error);
 
-  if (error != NULL)
+  if (error == NULL)
     {
       if (save_filename != filename)
 	{