NetworkPkg: Check for the max DHCP packet length before use it.

NetworkPkg: Check for the max DHCP packet length before use it.

This patch updates the PXE and HTTP boot driver to drop the input DHCP packet
if it exceed the maximum length.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-By: Wu Jiaxin <jiaxin.wu@intel.com>