message: Fix CommitList contexMenuEvent
  Solve issue by checking if data isValid before accessing data member isStarred / setStarred
diff: @@ -1469,7 +1469,7 @@ void CommitList::contextMenuEvent(QContextMenuEvent *event)
  // multiple selection
  bool anyStarred = false;
  foreach (const QModelIndex &index, selectionModel()->selectedIndexes()) {
  - if (index.data(CommitRole).value<git::Commit>().isStarred()) {
  + if (index.data(CommitRole).isValid() && index.data(CommitRole).value<git::Commit>().isStarred()) {
  anyStarred = true;
  break;
  }
  @@ -1477,6 +1477,7 @@ void CommitList::contextMenuEvent(QContextMenuEvent *event)
  menu.addAction(anyStarred ? tr("Unstar") : tr("Star"), [this, anyStarred] {
  foreach (const QModelIndex &index, selectionModel()->selectedIndexes())
  + if (index.data(CommitRole).isValid())
  index.data(CommitRole).value<git::Commit>().setStarred(!anyStarred);
  });
  