Fix segfault when discarding changes on open

When opening new schematic, the undo/redo list is invalidated.
CleanupSchematic uses it before presenting the new schematic to the
user, so we need to make sure that old data are not preserved here past
their pointer lifetimes

eeschema/files-io.cpp

@@ -152,6 +152,7 @@ bool SCH_EDIT_FRAME::OpenProjectFiles( const std::vector<wxString>& aFileSet, in
 
     // unload current project file before loading new
     {
+        ClearUndoRedoList();
         SetScreen( nullptr );
         m_toolManager->GetTool<EE_INSPECTION_TOOL>()->Reset( TOOL_BASE::MODEL_RELOAD );
         CreateScreens();