fixed bug 44168

discovered by oss-fuzz

It's a bug in the test itself :
ZSTD_compressBound() as an upper bound of the compress size
only works for data compressed "normally".
But in situations where many flushes are forcefully introduced,
this creates many more blocks,
each of which has a potential to increase the size by 3 bytes.
In extreme cases (lots of small incompressible blocks), the expansion can go beyond ZSTD_compressBound().

This situation is similar when using the CompressSequences() API
with Explicit Block Delimiters.
In which case, each explicit block acts like a deliberate flush.
When employed by a fuzzer, it's possible to generate scenarios like the one described above,
with tons of incompressible blocks of small sizes,
thus going beyond ZSTD_compressBound().

fix : when using Explicit Block Delimiters, use a larger bound, to account for this scenario.

lib/common/zstd_internal.h

@@ -311,8 +311,8 @@ typedef struct {
      * in the seqStore that has a value larger than U16 (if it exists). To do so, we increment
      * the existing value of the litLength or matchLength by 0x10000.
      */
-    ZSTD_longLengthType_e   longLengthType;
-    U32                     longLengthPos;  /* Index of the sequence to apply long length modification to */
+    ZSTD_longLengthType_e longLengthType;
+    U32                   longLengthPos;  /* Index of the sequence to apply long length modification to */
 } seqStore_t;
 
 typedef struct {

lib/compress/zstd_compress.c

@@ -6140,7 +6140,7 @@ ZSTD_compressSequences_internal(ZSTD_CCtx* cctx,
         FORWARD_IF_ERROR(blockSize, "Error while trying to determine block size");
         assert(blockSize <= remaining);
         ZSTD_resetSeqStore(&cctx->seqStore);
-        DEBUGLOG(5, "Working on new block. Blocksize: %zu", blockSize);
+        DEBUGLOG(5, "Working on new block. Blocksize: %zu (total:%zu)", blockSize, (ip - (const BYTE*)src) + blockSize);
 
         additionalByteAdjustment = sequenceCopier(cctx, &seqPos, inSeqs, inSeqsSize, ip, blockSize);
         FORWARD_IF_ERROR(additionalByteAdjustment, "Bad sequence copy");