Compression errors with zlib 1.2.10

[maksis]

After updating zlib to version 1.2.10, transfers randomly fail because of compression errors. Such errors happening on uploader's side won't reported anywhere, the upload just gets disconnected.

Based on some debugging with AirDC++, the actual location where the error happens in zlib is https://github.com/madler/zlib/blob/master/deflate.c#L595

I'm not sure whether that's a bug in zlib or something intended as there are also breaking changes in version 1.2.10: https://github.com/madler/zlib/issues/190 (avail_out is not originally zero when the call is made)

Setting compression level at https://sourceforge.net/p/dcplusplus/code/ci/default/tree/dcpp/ZUtils.cpp#l59 to a non-zero value seems to help with avoiding the issue.