zstdmt : fix : loading prefix from previous segments

There used to be a (very small) chance that
loading prefix from previous segment
would be confused with a real zstd dictionary.
For that to happen, the prefix needs to start
with the same value as dictionary magic.
That's 1 chance in 4 billions if all values have equal probability.
But in fact, since some values are more common (0x00000000 for example)
others are less common, and dictionary magic was selected to be one of them,
so probabilities are likely even lower.

Anyway, this risk is no down to zero
by adding a new CCtx parameter : ZSTD_p_forceRawDict

Current parameter policy : the parameter "stick" to its CCtx,
so any dictionary loading after ZSTD_p_forceRawDict is set
will be loaded in "raw" ("content only") mode,
even if CCtx is re-used multiple times with multiple different dictionary.
It's up to the user to reset this value differently if it needs so.

lib/compress/zstdmt_compress.c

@@ -236,8 +236,9 @@ void ZSTDMT_compressChunk(void* jobDescription)
         if (job->cdict) DEBUGLOG(3, "using CDict ");
         if (ZSTD_isError(initError)) { job->cSize = initError; goto _endJob; }
     } else {  /* srcStart points at reloaded section */
+        size_t const dictModeError = ZSTD_setCCtxParameter(job->cctx, ZSTD_p_forceRawDict, 1);  /* Force loading dictionary in "content-only" mode (no header analysis) */
         size_t const initError = ZSTD_compressBegin_advanced(job->cctx, job->srcStart, job->dictSize, job->params, 0);
-        if (ZSTD_isError(initError)) { job->cSize = initError; goto _endJob; }
+        if (ZSTD_isError(initError) || ZSTD_isError(dictModeError)) { job->cSize = initError; goto _endJob; }
         ZSTD_setCCtxParameter(job->cctx, ZSTD_p_forceWindow, 1);
     }
     if (!job->firstChunk) {  /* flush and overwrite frame header when it's not first segment */