Return NULL in wrapCompressedReadStream for compressed streams when ZLIB is disabled.

As the title suggests this changes wrapCompressedReadStream to return NULL when it gets an compressed stream, but zLib is not available.

My main motivation for this is that I found the former handling of just returning the stream a bit confusing. DefaultSaveFileManager::openForLoading would now return NULL when it encounters a compressed stream but no zLib support is available. This seems a better solution then returning the engine the compressed stream.

Anyway open for comments.