FirefoxCacheParser skips blocks

[joachimmetz]

https://github.com/log2timeline/plaso/blob/master/plaso/parsers/firefox_cache.py#L309

    # A request can span multiple blocks, so we use modulo.
    file_offset = file_object.get_offset() - offset
    _, remainder = divmod(file_offset, block_size)

    # Move reader to next candidate block. Include the null-byte skipped above.
    file_object.seek(block_size - remainder, os.SEEK_CUR)

does not account for a remainder of 0

[joachimmetz]

Pending changes in #2070

[joachimmetz]

Fixed with changes for migration