NetworkPkg: Remove ASSERT and use error handling in IpSecDxe

This patch is used to refine the code by removing ASSERT and
using error handling in IpSecDxe driver.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Cc: Yao Jiewen <jiewen.yao@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>

NetworkPkg/IpSecDxe/Ikev2/Info.c

@@ -2,7 +2,7 @@
   The Implementations for Information Exchange.
 
   (C) Copyright 2015 Hewlett-Packard Development Company, L.P.<BR>
-  Copyright (c) 2010, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2010 - 2016, Intel Corporation. All rights reserved.<BR>
 
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
@@ -47,7 +47,9 @@ Ikev2InfoGenerator (
   InfoContext  = NULL;
   IkeSaSession = (IKEV2_SA_SESSION *) SaSession;
   IkePacket    = IkePacketAlloc ();
-  ASSERT (IkePacket != NULL);
+  if (IkePacket == NULL) {
+    return NULL;
+  }
 
   //
   // Fill IkePacket Header.