* lily/music-iterator.cc (quit, do_quit): new function: break link

* lily/music-iterator.cc (quit, do_quit): new function: break link
to translator group explicitly. This used to be done from the
destructor of music-iterator. Now that iterators are GC'd, this is
no longer possible.  Add new function do_quit() to all applicable
derived classes.

* lily/lyric-combine-music-iterator.cc (construct_children): don't
unprotect objects.
(Lyric_combine_music_iterator): small typo bugfix.

* lily/sequential-iterator.cc (get_pending_events): don't delete
music-iterator.