fixes adaptation on srcSize

This patch restores capability for each file to receive adapted compression parameters depending on its size.

The bug breaking this feature was relatively silly :
setting a parameter with a value "0" is supposed to be a no-op.
Unfortunately, it would pin down compression parameters as if they were manually set,
preventing later automatic adaptation.

Unfortunately, I'm currently short of a test case that could check this situation and trigger an error.
Compression parameters selection between tableID 0,1,2,3 is largely internal,
leaving no trace to outside world, not even in frame header.

programs/fileio.c

@@ -446,7 +446,7 @@ static cRess_t FIO_createCResources(const char* dictFileName, int cLevel,
         DISPLAYLEVEL(5,"set nb threads = %u \n", g_nbThreads);
         CHECK( ZSTD_CCtx_setParameter(ress.cctx, ZSTD_p_nbThreads, g_nbThreads) );
         /* dictionary */
-        CHECK( ZSTD_CCtx_setPledgedSrcSize(ress.cctx, srcSize) );  /* just for dictionary loading, using good compression parameters */
+        CHECK( ZSTD_CCtx_setPledgedSrcSize(ress.cctx, srcSize) );  /* just for dictionary loading, for compression parameters adaptation */
         CHECK( ZSTD_CCtx_loadDictionary(ress.cctx, dictBuffer, dictBuffSize) );
         CHECK( ZSTD_CCtx_setPledgedSrcSize(ress.cctx, ZSTD_CONTENTSIZE_UNKNOWN) );  /* reset */