Question on Grammar-based mutation

1. If there are seeds in input folder in the beginning, it failed to run with this message:
   FATAL("Incorrectly encoded grammar sample");
   I found it is generated becase of if sentence as below:
   bool GrammarFuzzer::OutputFilter(Sample* original_sample, Sample* output_sample, ThreadContext* tc) {
   uint64_t string_size = ((uint64_t)original_sample->bytes);
   if (original_sample->size < (string_size + sizeof(string_size))) {
   FATAL("Incorrectly encoded grammar sample");
   }
   I dont understant exact meaning of "if (original_sample->size < (string_size + sizeof(string_size)))": why do you put this comparison here?

2. I began to run javascript fuzzer with empty files in input folder. it continues to generate samples in sample folder, but most (almost everything) is grammar/syntantically incorrect. so it could not pass the parsing process at all. Do you have any idea to solve this problem? In the Domato fuzzer, you put trycatch phrases for every sentence. Is any easy way to put trycatch phrases for sentence? or do you have other options to solve it?

Thank you for reading my issue.
Best.

Start with GrammarFuzzer::OutputFilter and reproduce both cases described: initial seeds in the input folder and an empty input folder for the JavaScript fuzzer. Read the surrounding sample encoding and parsing flow, then verify that startup handles existing seeds and generated samples are accepted or that their failure is clearly reported.