m_bHasActive must be false after last visible entry is removed

...otherwise, ExtensionBox_Impl::RecalcAll would crash in CalcActiveHeight(-1).

Change-Id: If399825951c79b821ed44ebefa19d1032ab4f850
(cherry picked from commit 661f7bdefc8215f742951727b93395d47293cf0e)
Reviewed-on: https://gerrit.libreoffice.org/7097
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>