Make parsing of articulation shorthands more robust. (issue 3664)

Allow articulation shorthands to be any post-event. Commit 6baa453 tried
to restrict them to ArticulationEvents, but actually allowed anything.

As opposed to previously, return a dummy PostEvents on error as
fallback rather than an incomplete ArticulationEvent causing followup
errors and a crash.

Signed-off-by: David Kastrup <dak@gnu.org>