MdeModulePkg/DxeNetLib: Add parameter check and ASSERT handling.

MdeModulePkg/DxeNetLib: Add parameter check and ASSERT handling.

* Library API should check the input parameters before use, or
  ASSERT to tell it has to meet some requirements. But in DxeNetLib,
  not all functions follows this rule.
* ASSERT shouldn't be used as error handling, add some handling code
  for errors.
* Add some ASSERT commence in function notes.

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>