Support decompression of compressed blocks of size ZSTD_BLOCKSIZE_MAX

[Cyan4973]

The wording of the Zstandard specification
allows valid compressed blocks to be sized exactly ZSTD_BLOCKSIZE_MAX bytes (128 KB).
This generally does not happen, since an uncompressed block would have same size but suffer no decompression speed cost.

Starting with this patch, the libzstd decoder will now accept decoding a compressed block of size ZSTD_BLOCKSIZE_MAX exactly.

Note though that decoders from reference libzstd before < v1.5.4 consider this edge case as an error.

As a consequence, avoid generating compressed blocks of size ZSTD_BLOCKSIZE_MAX for broader compatibility with the ecosystem of deployed zstd decoders.
Note that the reference libzstd encoder never produces compressed blocks of size ZSTD_BLOCKSIZE_MAX, and will continue to do so for the foreseeable future, both to preserve compatibility with older decoders, and because it doesn't make sense to produce compressed blocks of such size (as opposed to uncompressed blocks).

The impact of this modification to the ecosystem is expected to be none. This is mostly meant to respect the letter of the RFC8878 specification.

Thanks to @SmitaKumar for reporting this issue in #2577