NetworkPkg: Avoid potential NULL pointer dereference

NetworkPkg: Avoid potential NULL pointer dereference

The commit of 6b16c9e7 removes ASSERT and use error handling
in IpSecDxe driver, but may cause the potential NULL pointer
dereference. So, this patch is used to avoid NULL pointer
dereference.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>