NetworkPkg: Fix unspecified address use case in IpsecConfig

NetworkPkg: Fix unspecified address use case in IpsecConfig

This patch is used to fix unspecified address use case in
ConstructSpdIndexer() function. Indexer->Name for
ConstructSpdIndexer is unspecified, that will be a problem
for UnicodeStrToAsciiStr.

This patch also refine the code by removing ASSERT and user
error handling.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Zeng Star <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Zeng Star <star.zeng@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>