ShellPkg/UefiDpLib: Add check to avoid NULL pointer deference

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

ShellPkg/Library/UefiDpLib/Dp.c

@@ -272,7 +272,7 @@ ShellCommandRunDp (
   //      EndCount = Value counter counts to before it needs to be reset
   //
   Status = EfiGetSystemConfigurationTable (&gPerformanceProtocolGuid, (VOID **) &PerformanceProperty);
-  if (EFI_ERROR (Status)) {
+  if (EFI_ERROR (Status) || (PerformanceProperty == NULL)) {
     ShellPrintHiiEx (-1, -1, NULL, STRING_TOKEN (STR_PERF_PROPERTY_NOT_FOUND), gDpHiiHandle);
     goto Done;
   }