Add uncompressed chunk padding

[Starwarsfan2099]

On a strange edge case, the final chunk of a file can be larger than chunk_size when compressed and smaller than chunk_size when uncompressed. If the compressed chunk is larger than chunk_size, the uncompressed chunk is used. However, since the uncompressed final chunk is smaller than chunk_size, when extracting, pyaff4 attempts to decompress the uncompressed chunk leading to a snappy.UncompressError error. Adding a check and padding any uncompressed chunk less than chunk_size allows for proper extraction of the files from the container and no errors. This was found when a single file out of 34,274 files created this error.

[blschatz]

Thanks @Starwarsfan2099. Good catch!