bug#23899: 24.5; mouse-on-link-p breaks for positions

[sbaugh]

The documentation for mouse-on-link-p says:
    POS must be a buffer position in the current buffer or a mouse
    event location in the selected window (see `event-start').
However, doing M-: (mouse-on-link-p (point)) fails with a type
error. And, in general, passing a buffer position to mouse-on-link-p
fails with a type error.

This appears to be due to this section of mouse-on-link-p:
    (let ((area (posn-area pos)))
      (when area
        (key-binding (vector area 'follow-link) nil t pos)))

I would recommend wrapping this in a (and (consp pos) ...) to avoid
running posn-area on buffer positions. The rest of the function works
fine, this part was added later.