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 f9bbb8d9c3f065faba9f266cf4e731fe2ca70c4d.
    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>