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>

MdeModulePkg/Universal/Network/UefiPxeBcDxe/PxeBcSupport.c

@@ -132,11 +132,10 @@ CvtNum (
 {
   UINTN Remainder;
 
-  while (Length > 0) {
+  for (; Length > 0; Length--) {
     Remainder = Number % 10;
     Number /= 10;
-    Length--;
-    Buffer[Length] = (UINT8) ('0' + Remainder);
+    Buffer[Length - 1] = (UINT8) ('0' + Remainder);
   }
 }