MdeModulePkg DxeS3BootScriptLib: Check (mSmst != NULL) before freeing SMRAM

MdeModulePkg DxeS3BootScriptLib: Check (mSmst != NULL) before freeing SMRAM

There is static scan tool reports BootScriptSave.c:628:'mSmst' is
explicitly dereferenced.

The patch is to check (mSmst != NULL) before freeing SMRAM at
BootScriptSave.c:628.

Cc: Shumin Qiu <shumin.qiu@intel.com>
Cc: Jiewen Yao <jiewen.yao@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>