Here is an update patch, which corrects a typo in the previous patch, and adds a test to test_zlib.
The test uses a memory-mapped sparse file, so it gets skipped on systems without mmap. The alternative would be to allocate a 4+GB buffer of ordinary memory, causes heavy swapping on my machine (4GB of RAM). The test also gets skipped on 32-bit builds, where the address space is too small for this bug to arise.
I'm not sure whether the test can count on the created file actually being sparse, so I had the test require the 'largefile' resource, to be on the safe side.