MdeModulePkg BrotliLib: Fix the regression logic issue in loop

In V2, change logic to avoid use mtf[-1] style to get value.

Roll back to previous logic, and use point + offset to get byte value.

Cc: Bell Song <binx.song@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bell Song <binx.song@intel.com>