[fix] Add missing bounds checks during compression

* The block splitter missed a bounds check, so when the buffer is too small it
  passes an erroneously large size to `ZSTD_entropyCompressSeqStore()`, which
  can then write the compressed data past the end of the buffer. This is a new
  regression in v1.5.0 when the block splitter is enabled. It is either enabled
  explicitly, or implicitly when using the optimal parser and `ZSTD_compress2()`
  or `ZSTD_compressStream*()`.
* `HUF_writeCTable_wksp()` omits a bounds check when calling
  `HUF_compressWeights()`. If it is called with `dstCapacity == 0` it will pass
  an erroneously large size to `HUF_compressWeights()`, which can then write
  past the end of the buffer. This bug has been present for ages. However, I
  believe that zstd cannot trigger the bug, because it never calls
  `HUF_compress*()` with `dstCapacity == 0` because of [this check][1].

Credit to: Oss-Fuzz

[1]: https://github.com/facebook/zstd/blob/89127e5ee2f3c1e141668fa6d4ee91245f05d132/lib/compress/zstd_compress_literals.c#L100

lib/compress/zstd_compress.c

@@ -3486,6 +3486,7 @@ static size_t ZSTD_compressSeqStore_singleBlock(ZSTD_CCtx* zc, seqStore_t* const
     if (isPartition)
         ZSTD_seqStore_resolveOffCodes(dRep, cRep, seqStore, (U32)(seqStore->sequences - seqStore->sequencesStart));
 
+    RETURN_ERROR_IF(dstCapacity < ZSTD_blockHeaderSize, dstSize_tooSmall, "Block header doesn't fit");
     cSeqsSize = ZSTD_entropyCompressSeqStore(seqStore,
                 &zc->blockState.prevCBlock->entropy, &zc->blockState.nextCBlock->entropy,
                 &zc->appliedParams,