Well, here's a simple thing you can check: Does your zlib dll link to _lseek or _lseeki64? The first one uses a 32-bit offset. Wine's implementation (http://source.winehq.org/source/dlls/msvcrt/file.c#L1090) expands that to 64-bit and later truncates the file offset to 32-bit. For a file larger than 2 GB, that could account for the large negative value you're seeing.
And since this would only matter in cases where zlib uses lseek (the first time through I guess it wouldn't, as it has to read the whole everything up to the offset you give at least once) and is at least 2 GB into the file, that might also explain why it doesn't fail initially.
But without really digging into the zlib code, all I can do it speculate.
I should probably also check coapp's build of zlib sometime.