MdeModulePkg: Use IsZeroGuid API for zero GUID checking

Instead of comparing a GUID with gZeroGuid via the CompareGuid API, the
commit uses the IsZeroGuid API to check if the given GUID is a zero GUID.

Cc: Feng Tian <feng.tian@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Feng Tian <feng.tian@Intel.com>

MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.c

@@ -14,7 +14,7 @@
   VariableServiceSetVariable(), VariableServiceQueryVariableInfo(), ReclaimForOS(),
   SmmVariableGetStatistics() should also do validation based on its own knowledge.
 
-Copyright (c) 2010 - 2015, 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
 which accompanies this distribution.  The full text of the license may be found at
@@ -35,7 +35,6 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 #include <Library/SmmMemLib.h>
 
 #include <Guid/SmmVariableCommon.h>
-#include <Guid/ZeroGuid.h>
 #include "Variable.h"
 
 extern VARIABLE_INFO_ENTRY                           *gVariableInfo;
@@ -373,7 +372,7 @@ SmmVariableGetStatistics (
 
   CopyGuid (&VendorGuid, &InfoEntry->VendorGuid);
 
-  if (CompareGuid (&VendorGuid, &gZeroGuid)) {
+  if (IsZeroGuid (&VendorGuid)) {
     //
     // Return the first variable info
     //

MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf

@@ -114,7 +114,6 @@
   ## SOMETIMES_CONSUMES   ## Variable:L"VarErrorFlag"
   ## SOMETIMES_PRODUCES   ## Variable:L"VarErrorFlag"
   gEdkiiVarErrorFlagGuid
-  gZeroGuid                                     ## SOMETIMES_CONSUMES   ## GUID
 
 [Pcd]
   gEfiMdeModulePkgTokenSpaceGuid.PcdFlashNvStorageVariableSize       ## CONSUMES