S&R: fix crash when closing search tabs
Always call QTabWidget::removeTab() so that the widget represented by that tab is removed from the underlying QStackedWidget, otherwise the code could try accessing a MatchModel object that has already been deleted as its parent Results object has been deleted.
Only set m_curResults to nullptr if it belongs to the tab being closed, e.g. if you have two or more search tabs, you can click the close button of a tab other than the current one.