MdeModulePkg/DxeIpIoLib: Fixed the error ASSERT incorrectly used in IpIoAddIp().

* In DxeIpIo, an ASSERT check is incorrectly used in IpIoAddIp(), which result
  debug image hang when this API is called, this patch is to fix this issue.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Fu Siyuan <siyuan.fu@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>