Only attempt a graceful shutdown when there's a STM hook

For Wii graceful shutdown to work, the emulated software has to open
the STM event hook and install a hook. Without this, there is no way
to inform them about the shutdown, so trying to do a graceful shutdown
and requiring the use of the shutdown fallback (exiting a second time
to force) is pointless.

Source/Core/Core/IPC_HLE/WII_IPC_HLE_Device_stm.cpp

@@ -135,6 +135,11 @@ IPCCommandResult CWII_IPC_HLE_Device_stm_eventhook::IOCtl(u32 command_address)
   return GetNoReply();
 }
 
+bool CWII_IPC_HLE_Device_stm_eventhook::HasHookInstalled() const
+{
+  return s_event_hook_address != 0;
+}
+
 void CWII_IPC_HLE_Device_stm_eventhook::TriggerEvent(const u32 event) const
 {
   if (!m_Active || s_event_hook_address == 0)

Source/Core/Core/IPC_HLE/WII_IPC_HLE_Device_stm.h

@@ -59,6 +59,7 @@ public:
   IPCCommandResult Close(u32 command_address, bool force) override;
   IPCCommandResult IOCtl(u32 command_address) override;
 
+  bool HasHookInstalled() const;
   void ResetButton() const;
   void PowerButton() const;