Panics discovered by fuzzer

[Shnatsel]

I have decoded the AFL-generated set of exotic JPEGs when fuzzing libjpeg-turbo, as well as the files generated by AFL when targeting the jpeg-decoder crate. Decoding these files causes multiple panics in zune-jpeg.

The set of files triggering the panics is attached: jpeg_fuzzing_seeds.tar.gz

Aside of being interesting test cases, these files can also be used to kickstart fuzzing - simply put them into fuzz/corpus/decode_buffer and run the fuzzer as usual. This should provide much better coverage than starting from scratch.