fixed : Visual build compressing stdin with multi-threading enabled fails

It was multiple reasons stacked :
- Visual use a different code path, because ZSTD_NEWAPI is not defined
- fileio.c sends `0` as `pledgedSrcSize` to mean `ZSTD_CONTENTSIZE_UNKNOWN`  (fixed)
- ZSTDMT_resetCCtx() interpreted `0` as "empty" instead of "unknown" (fixed)

lib/compress/zstdmt_compress.c

@@ -838,9 +838,12 @@ size_t ZSTDMT_initCStream_usingCDict(ZSTDMT_CCtx* mtctx,
 
 
 /* ZSTDMT_resetCStream() :
- * pledgedSrcSize is optional and can be zero == unknown */
+ * pledgedSrcSize can be zero == unknown (for the time being)
+ * prefer using ZSTD_CONTENTSIZE_UNKNOWN,
+ * as `0` might mean "empty" in the future */
 size_t ZSTDMT_resetCStream(ZSTDMT_CCtx* zcs, unsigned long long pledgedSrcSize)
 {
+    if (!pledgedSrcSize) pledgedSrcSize = ZSTD_CONTENTSIZE_UNKNOWN;
     if (zcs->params.nbThreads==1)
         return ZSTD_resetCStream(zcs->cctxPool->cctx[0], pledgedSrcSize);
     return ZSTDMT_initCStream_internal(zcs, NULL, 0, ZSTD_dm_auto, 0, zcs->params,

lib/compress/zstdmt_compress.h

@@ -50,7 +50,7 @@ ZSTDLIB_API size_t ZSTDMT_compressCCtx(ZSTDMT_CCtx* mtctx,
 /* ===   Streaming functions   === */
 
 ZSTDLIB_API size_t ZSTDMT_initCStream(ZSTDMT_CCtx* mtctx, int compressionLevel);
-ZSTDLIB_API size_t ZSTDMT_resetCStream(ZSTDMT_CCtx* mtctx, unsigned long long pledgedSrcSize);    /**< pledgedSrcSize is optional and can be zero == unknown */
+ZSTDLIB_API size_t ZSTDMT_resetCStream(ZSTDMT_CCtx* mtctx, unsigned long long pledgedSrcSize);  /**< if srcSize is not known at reset time, use ZSTD_CONTENTSIZE_UNKNOWN. Note: for compatibility with older programs, 0 means the same as ZSTD_CONTENTSIZE_UNKNOWN, but it may change in the future, to mean "empty" */
 
 ZSTDLIB_API size_t ZSTDMT_compressStream(ZSTDMT_CCtx* mtctx, ZSTD_outBuffer* output, ZSTD_inBuffer* input);