Fix backtrace on doubleclick in vterm

[sunlin7]

Hi,

Backtrace happened when I double click in a vterm,
it caused by (mouse-set-point event..) return nil for "double-mouse-1" event,
then lead the count-words failed with nil.

Debugger entered--Lisp error: (wrong-type-argument integer-or-marker-p nil)
count-words(nil 370)
(= (count-words pt (point-max)) 0)
(if (= (count-words pt (point-max)) 0) (vterm-reset-cursor-point) pt)
(let ((pt (mouse-set-point event promote-to-region))) (if (= (count-words pt (point-max)) 0) (vterm-reset-cursor-point) pt))
vterm-mouse-set-point((double-mouse-1 (#<window 335 on Default-multivterm-1> 193 (1273 . 277) 105236234 nil 193 (63 . 6) nil (13 . 19) (20 . 43)) 2) 1)
funcall-interactively(vterm-mouse-set-point (double-mouse-1 (#<window 335 on Default-multivterm-1> 193 (1273 . 277) 105236234 nil 193 (63 . 6) nil (13 . 19) (20 . 43)) 2) 1)
command-execute(vterm-mouse-set-point)

This patch will fix the backtrace.
Please help review and merge the changes.