vt: fix double-click word selection for last word on line

Previously when double-clicking on the last word on a line we would
select from the beginning of the word to the cursor position, because
we searched forward for a space character to find the end of a word.
Now, use the end of the line if we do not find a space.

PR: 261553
Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34339