Issue 4419: Engraving ends too early

Issue 4419: Engraving ends too early

This is a followup on the solution for issue 2010 that was too eager
killing off unrelated iterators when an iterator in the vicinity of a
Lyric_combine_music_iterator died.

The salient point is to have Simultaneous_music_iterator::process_music
check for pending_moment () going from finite to infinite when
iterating, signifying the loss of an iterator defining an end point.

It happens to also fix issue 4339.