MdeModulePkg/UefiPxeBcDxe: Refine the CvtNum function logic

MdeModulePkg/UefiPxeBcDxe: Refine the CvtNum function logic

This commit refines the logic for the CvtNum function. It avoids using the
decrement operator '--' for array index to prevent possible mis-reports by
static code checkers.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>