Fix #781.

Fix #781.

- don't dereference non-existent event-cause, which may occur if the
Ambitus_engraver is erroneously added to both the Staff and Voice contexts:
in such a situation, Ambitus_engraver::acknowledge_note_head () will acknowledge
an AmbitusNoteHead, which has no event-cause.
(cherry picked from commit d511b69c1cb8836bc0b7b49b9f396cf06fef19ac)