Fixed a nasty corruption bug

recently introduce into the new dictionary mode.
The bug could be reproduced with this command :
./zstreamtest -v --opaqueapi --no-big-tests -s4092 -t639

error was in function ZSTD_count_2segments() :
the beginning of the 2nd segment corresponds to prefixStart
and not the beginning of the current block (istart == src).
This would result in comparing the wrong byte.

lib/compress/zstd_compress.c

@@ -581,6 +581,7 @@ size_t ZSTD_CCtxParam_getParameter(
 size_t ZSTD_CCtx_setParametersUsingCCtxParams(
         ZSTD_CCtx* cctx, const ZSTD_CCtx_params* params)
 {
+    DEBUGLOG(4, "ZSTD_CCtx_setParametersUsingCCtxParams");
     if (cctx->streamStage != zcss_init) return ERROR(stage_wrong);
     if (cctx->cdict) return ERROR(stage_wrong);
 
@@ -1226,6 +1227,8 @@ static size_t ZSTD_resetCCtx_usingCDict(ZSTD_CCtx* cctx,
                         && ZSTD_equivalentCParams(cctx->appliedParams.cParams,
                                                   cdict->cParams);
 
+    DEBUGLOG(4, "ZSTD_resetCCtx_usingCDict (pledgedSrcSize=%u)", (U32)pledgedSrcSize);
+
 
     {   unsigned const windowLog = params.cParams.windowLog;
         assert(windowLog != 0);