Fix XZBlock padding calculation when its stream's starting position % 4 != 0

[Morilli]

• closes Padding calculation wrong when starting position % 4 != 0 #873

I've also removed the unused _bytesRead field that seems to have last been touched in #300.

[Morilli]

Hm, test failures seem to occur because XZBlock is instantiated with a (RewindableStream) stream that throws an exception when accessing its Position... not sure how valid that even is, considering reading the block would eventually fail anyways because SkipPadding already accesses the stream's position before this change.

[adamhathcock]

Thanks!