Fix parallel deflate

Fix a bug in parallel deflate with big files where output would be truncated ending in invalid zip file (and crc errors on extracting).

You can read about the bug here and how to reproduce it: https://dotnetzip.codeplex.com/workitem/14087
The fix was provided by **rhpainte**, so all credits to him.

By the way there seem to be some issues with mixed line endings in this repository. I couldn't get it to work with the github client on windows.