MdeModulePkg/DxeNetLib: Fix an error in packet length counting.

MdeModulePkg/DxeNetLib: Fix an error in packet length counting.

* In old implementation, the operation len-- assumes AsciiSPrint()
  has counted NULL terminator, and it's not correct. This patch is
  to fix this issue.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wang Fan <fan.wang@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>