EndListeningContext holds BroadcasterStoreType iterators in its
ColumnBlockPositionSet and collects broadcasters to purge them at the
end. Removing broadcasters from ScColumn::maBroadcasters in between
invalidates the iterators. Hence calling the "normal" EndListening()
that removes a broadcaster when all listeners are gone while an
EndListeningContext is in use is bad.

Change-Id: Ibdd88469e91e6173ceff1f391c23ef7cb7c6f596