[emacs mode] Avoid wrong-type-argument error in completion callback

The error is:

    Error in post-command-hook (completion-in-region--postch):
    (wrong-type-argument number-or-marker-p closure)

and is triggered by the completion-in-region-mode-predicate bound in
completion-at-point.

Commit d9fc8c16 prevents completion-in-region-mode from being entered
via tern-do-complete, but is ineffective when completion-at-point is
invoked by some other means, e.g. after the *Completions* buffer has
been displayed.