LibGUI+FileManager: Fix forgetting to map sorting proxy model indexes

Also assert indexes are valid in a few more places.

Finally fixes https://github.com/SerenityOS/serenity/issues/1440 and
https://github.com/SerenityOS/serenity/issues/2787 :^)

Libraries/LibGUI/FileSystemModel.cpp

@@ -332,6 +332,7 @@ const FileSystemModel::Node& FileSystemModel::node(const ModelIndex& index) cons
 {
     if (!index.is_valid())
         return *m_root;
+    ASSERT(index.internal_data());
     return *(Node*)index.internal_data();
 }