Don't allow undo record to get pushed during an undo.

When doing a schematic cleanup, for instance, we were breaking wires
and pushing undo records on to the stack.  Needless to say, this was
pretty unexpected for the rest of the undo framework and caused all
kinds of mayhem (including crashes in some cases).

Fixes https://gitlab.com/kicad/code/kicad/issues/8704

eeschema/tools/sch_editor_control.cpp

@@ -1166,18 +1166,23 @@ int SCH_EDITOR_CONTROL::Undo( const TOOL_EVENT& aEvent )
 
     /* Get the old list */
     PICKED_ITEMS_LIST* List = m_frame->PopCommandFromUndoList();
+    size_t num_undos = m_frame->m_undoList.m_CommandsList.size();
 
     /* Undo the command */
     m_frame->PutDataInPreviousState( List );
 
-    /* Put the old list in RedoList */
-    List->ReversePickersListOrder();
-    m_frame->PushCommandToRedoList( List );
-
     m_frame->SetSheetNumberAndCount();
     m_frame->TestDanglingEnds();
     m_frame->OnPageSettingsChange();
 
+    // If we modified anything during cleanup we don't want it going on the undolist
+    while( m_frame->m_undoList.m_CommandsList.size() > num_undos )
+        delete m_frame->PopCommandFromUndoList();
+
+    // Now push the old command to the RedoList
+    List->ReversePickersListOrder();
+    m_frame->PushCommandToRedoList( List );
+
     m_toolMgr->GetTool<EE_SELECTION_TOOL>()->RebuildSelection();
 
     m_frame->SyncView();