MdeModulePkg/SmmCore: Fix hang due to already-freed memory deference

MdeModulePkg/SmmCore: Fix hang due to already-freed memory deference

SmiHandlerUnRegister() validates the DispatchHandle by checking
whether the first 32bit matches to a certain signature
(SMI_HANDLER_SIGNATURE).
But if a caller calls *UnRegister() twice and the memory freed by
first call still contains the signature, the second call may hang.

The patch fixes this issue by locating the DispatchHandle
in all SMI handlers, instead of checking the signature.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>