spelling: correctly adjust the length of a single-line region

When the marked region covers only a single line (or a part of it),
its new endpoint is not simply the length of the last line of the
spell-checked text, but instead the old endpoint plus the /change/
in length.

This fixes https://savannah.gnu.org/bugs/?50316.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>
1 file changed