[Patch 5/5] NetworkPkg: Update PXE driver to check for NULL pointer before use it.

[Fu Siyuan <siyuan.fu@intel.com>]

This patch is to fix the issue that dereferencing of "This" (EFI_LOAD_FILE_PROTOCOL)
in EfiPxeLoadFile() is happening before the NULL check.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Cc: Wang Fan <fan.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
---
 NetworkPkg/UefiPxeBcDxe/PxeBcImpl.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/NetworkPkg/UefiPxeBcDxe/PxeBcImpl.c b/NetworkPkg/UefiPxeBcDxe/PxeBcImpl.c
index dc77d256f8..93f3bfa5ba 100644
--- a/NetworkPkg/UefiPxeBcDxe/PxeBcImpl.c
+++ b/NetworkPkg/UefiPxeBcDxe/PxeBcImpl.c
@@ -1,7 +1,7 @@
 /** @file
   This implementation of EFI_PXE_BASE_CODE_PROTOCOL and EFI_LOAD_FILE_PROTOCOL.
 
-  Copyright (c) 2007 - 2017, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2007 - 2018, 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
@@ -2349,26 +2349,22 @@ EfiPxeLoadFile (
   EFI_STATUS                  Status;
   EFI_STATUS                  MediaStatus;
 
-  if (FilePath == NULL || !IsDevicePathEnd (FilePath)) {
+  if (This == NULL || BufferSize == NULL || FilePath == NULL || !IsDevicePathEnd (FilePath)) {
     return EFI_INVALID_PARAMETER;
   }
   
-  VirtualNic = PXEBC_VIRTUAL_NIC_FROM_LOADFILE (This);
-  Private    = VirtualNic->Private;
-  PxeBc      = &Private->PxeBc;
-  UsingIpv6  = FALSE;
-  Status     = EFI_DEVICE_ERROR;
-
-  if (This == NULL || BufferSize == NULL) {
-    return EFI_INVALID_PARAMETER;
-  }
-
   //
   // Only support BootPolicy
   //
   if (!BootPolicy) {
     return EFI_UNSUPPORTED;
   }
+  
+  VirtualNic = PXEBC_VIRTUAL_NIC_FROM_LOADFILE (This);
+  Private    = VirtualNic->Private;
+  PxeBc      = &Private->PxeBc;
+  UsingIpv6  = FALSE;
+  Status     = EFI_DEVICE_ERROR;
 
   //
   // Check media status before PXE start
-- 
2.13.0.windows.1