fix initCStream_advanced() for fast strategies

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)