Fix overflow bug on bogus code lengths

[oyvindln]

Fixes #14

When the set of code lengths for the huffman codes encoding the main huffman tables contained a set of lengths that could not create a set of valid codes, the overflow discovered in #14 happened.

This PR checks for the overflow, and returns an error instead of overflowing.

Also added a test to check for the issue.

I don't know if the error message is descriptive enough, or if it ought to be changed.

[oyvindln]

Okay, seems overflowing_add was added in 1.7. I suppose the easiest way to fix this would be to check for code == 255 instead (The suggestion by the compiler on 1.0 doesn't seem to work with current rust). Any specific reason you need it to work with a version of rust this old?

[eddyb]

@oyvindln Can't you use checked_add?

[oyvindln]

Yeah that's probably cleaner anyhow.

[oyvindln]

Okay, travis suggests it's working on 1.0 now.

[eddyb]

Thanks!