OSS-Fuzz coverage improvment

[Segwaz]

I submitted a PR a few weeks ago that significantly improves the fuzzing coverage of your project, with a focus on parsers. While it occasionally uses internal APIs due to the unconventional nature of this type of testing, I am reasonably confident in its stability as I ran the fuzz targets for an extended period at home and took the time to read the related source code.

Would you be able to review it when you have some time? You can find it here: google/oss-fuzz#12852