[zstd][leak] Avoid memory leak on early return of ZSTD_generateSequence

Sanity checks on a few of the context parameters (i.e. workers and block size)
may prompt an early return on ZSTD_generateSequences.

Allocating the destination buffer past those return points avoids a potential
memory leak.

This patch should fix issue facebook#4112.

lib/compress/zstd_compress.c

@@ -3458,7 +3458,7 @@ size_t ZSTD_generateSequences(ZSTD_CCtx* zc, ZSTD_Sequence* outSeqs,
  size_t outSeqsSize, const void* src, size_t srcSize)
 {
  const size_t dstCapacity = ZSTD_compressBound(srcSize);
- void* dst = ZSTD_customMalloc(dstCapacity, ZSTD_defaultCMem);
+ void* dst; /* Make C90 happy. */
  SeqCollector seqCollector;
  {
  int targetCBlockSize;
@@ -3471,6 +3471,7 @@ size_t ZSTD_generateSequences(ZSTD_CCtx* zc, ZSTD_Sequence* outSeqs,
  RETURN_ERROR_IF(nbWorkers != 0, parameter_unsupported, "nbWorkers != 0");
  }
 
+ dst = ZSTD_customMalloc(dstCapacity, ZSTD_defaultCMem);
  RETURN_ERROR_IF(dst == NULL, memory_allocation, "NULL pointer!");
 
  seqCollector.collectSequences = 1;