NetworkPkg: Add assert for buffer pointer from DHCP driver.

NetworkPkg: Add assert for buffer pointer from DHCP driver.

The PxeBcDhcp4CallBack() is provided for DHCP driver to invoke packet check
during DHCP process, the DHCP driver should make sure Packet and NewPacket
has meaningful value. This patch adds assert for this.

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