Auto_beam_engraver: fix context lifetime issue
Allow assigning and dereferencing a Context_handle
like a pointer.
Explain why handle owners must explicitly nullify them. assert ()
that
they have done so before the handle is destroyed. This should help
avoid repeating the mistake of issue #6402 (closed).
Nullify a handle in auto-beam-engraver.cc to avoid failing the new assertion.
Edited by Dan Eble