Arithmetic overflow found while fuzzing

[killercup]

I let this fuzzer run for a while (> 250k iterations), and it discovered:

thread '<unnamed>' panicked at 'attempt to add with overflow'
[…]
jpeg_decoder::huffman::derive_huffman_codes in src/huffman.rs:277

You can find the full log as well as the used input here: https://gist.github.com/killercup/d836f174582ee5d90403e9920d893be2

[kaksmet]

Thanks for fuzzing and reporting, I'll take a look at it over the weekend.