Fixed the issue that IPv6 network stack assert infrequently – assert once every 300...

Fixed the issue that IPv6 network stack assert infrequently – assert once every 300~400 PXE boots.

Signed-off-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ouyang Qian <qian.ouyang@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14422 6f19259b-4bc3-4df7-8a09-765794883524