Double click in terminal results in error

[sid-code]

Steps to reproduce:

1. Open vterm with M-x vterm
2. Double click anywhere to select a word
3. Observe error: vterm-mouse-set-point: Wrong type argument: integer-or-marker-p, nil

This doesn't cause any issues as the text still gets selected.

[sid-code]

This is addresesd by #715