ShellPkg/Dp: Add null pointer check

Cc: Liming Gao <liming.gao@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

ShellPkg/DynamicCommand/DpDynamicCommand/Dp.c

@@ -257,7 +257,7 @@ GetBootPerformanceTable (
                &AcpiTable
                  );
   }
-  if (EFI_ERROR(Status)) {
+  if (EFI_ERROR(Status) || AcpiTable == NULL) {
     return Status;
   }