NetworkPkg:Fix NULL pointer dereference issues.

NetworkPkg:Fix NULL pointer dereference issues.

Revise some errors that some Null pointers may be dereferenced.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18961 6f19259b-4bc3-4df7-8a09-765794883524