fix initCStream_advanced() for fast strategies

Compression ratio of fast strategies (levels 1 & 2)
was seriously reduced, due to accidental disabling of Literals compression.

Credit to @QrczakMK, which perfectly described the issue, and implementation details,
making the fix straightforward.

Example : initCStream with level 1 on synthetic sample P50 :
Before : 5,273,976 bytes
After  : 3,154,678 bytes
ZSTD_compress (for comparison) : 3,154,550

Fix #1787.

To follow : refactor the test which was supposed to catch this issue (and failed)

tests/fullbench.c

@@ -45,7 +45,6 @@
 #define NBLOOPS    6
 #define TIMELOOP_S 2
 
-#define KNUTH      2654435761U
 #define MAX_MEM    (1984 MB)
 
 #define DEFAULT_CLEVEL 1
@@ -722,7 +721,7 @@ static int usage_advanced(const char* exename)
     DISPLAY( "\nAdvanced options :\n");
     DISPLAY( " -b#    : test only function # \n");
     DISPLAY( " -l#    : benchmark functions at that compression level (default : %i)\n", DEFAULT_CLEVEL);
-    DISPLAY( " --zstd : custom parameter selection. Format same as zstdcli \n");
+    DISPLAY( "--zstd= : custom parameter selection. Format same as zstdcli \n");
     DISPLAY( " -P#    : sample compressibility (default : %.1f%%)\n", COMPRESSIBILITY_DEFAULT * 100);
     DISPLAY( " -B#    : sample size (default : %u)\n", (unsigned)kSampleSizeDefault);
     DISPLAY( " -i#    : iteration loops [1-9](default : %i)\n", NBLOOPS);