MdeModulePkg PiSmmCore: Ensure SxDispatch not NULL before dereferenced

Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>

MdeModulePkg/Core/PiSmmCore/PiSmmCore.c

@@ -321,7 +321,7 @@ SmmReadyToBootHandler (
              NULL,
              (VOID **)&SxDispatch
              );
-  if (!EFI_ERROR (Status)) {
+  if (!EFI_ERROR (Status) && (SxDispatch != NULL)) {
     //
     // Register a S3 entry callback function to
     // determine if it will be during S3 resume.