mport index decompression can leave partial or corrupt index data after zstd failures

mport is the MidnightBSD Package Manager. Prior to 2.7.8, libmport/util.c did not make every truncated, corrupt, or failed zstd stream fatal in mport_decompress_zstd(), and libmport/fetch.c did not consistently propagate those failures to index-fetch callers. A malicious or faulty mirror could supply compressed package index data that caused ZSTD_decompressStream() or an output write to fail while leaving partial index output available for later use, resulting in package-index integrity loss or denial of service. This issue is fixed in version 2.7.8.