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.