DolphinWX: Don't bind context menu when GC filesystem is invalid

If this isn't done, it's possible to select Extract All Files
and make Dolphin dereference a nullptr m_filesystem.

Source/Core/DolphinWX/ISOProperties/FilesystemPanel.h

@@ -39,7 +39,7 @@ private:
   void CreateGUI();
   void BindEvents();
 
-  void PopulateFileSystemTree();
+  bool PopulateFileSystemTree();
 
   void OnRightClickTree(wxTreeEvent&);
   void OnExtractFile(wxCommandEvent&);