MdeModulePkg AcpiTableDxe: Use Rsdt to check against NULL

Some static scan tool may regard CurrentRsdtEntry to be potentially
referenced to NULL pointer if CurrentRsdtEntry == NULL is used in
the right above if condition judgment.

        CopyMem (CurrentRsdtEntry, CurrentRsdtEntry + 1, (*NumberOfTableEntries - Index) * sizeof (UINT32));

It is introduced by commit f9bbb8d9c3.
To avoid it and have same style with
"((Xsdt == NULL) || CurrentTablePointer64 == (UINT64) (UINTN) Table->Table)",
use Rsdt instead of CurrentRsdtEntry to check against NULL.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Shumin Qiu <shumin.qiu@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Shumin Qiu <shumin.qiu@intel.com>

MdeModulePkg/Universal/Acpi/AcpiTableDxe/AcpiTableProtocol.c

@@ -1,7 +1,7 @@
 /** @file
   ACPI Table Protocol Implementation
 
-  Copyright (c) 2006 - 2015, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2006 - 2016, Intel Corporation. All rights reserved.<BR>
   Copyright (c) 2016, Linaro Ltd. All rights reserved.<BR>
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
@@ -1132,7 +1132,7 @@ RemoveTableFromRsdt (
     //
     // Check if we have found the corresponding entry in both RSDT and XSDT
     //
-    if ((CurrentRsdtEntry == NULL || *CurrentRsdtEntry == (UINT32) (UINTN) Table->Table) &&
+    if (((Rsdt == NULL) || *CurrentRsdtEntry == (UINT32) (UINTN) Table->Table) &&
         ((Xsdt == NULL) || CurrentTablePointer64 == (UINT64) (UINTN) Table->Table)
         ) {
       //