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

Previously when double-clicking on the first word on a line we would
select from the cursor position to the end of the word, not from the
beginning of the line. This is because we searched backward for a
space to mark the beginning of a word.

Now, use the beginning of the line if we do not find a space.

PR: 261553
Reported by: Stefan B.
MFC after: 1 week
Sponsored by: The FreeBSD Foundation