mMailboxPointer is used before it is initialization. This issue only happens
when SmmDebugAgent is initialized without PeiDebugAgent or DxeDebugAgent
initialization. The fix is to use Mailbox instead.
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>