Fix crash when selecting text with changes

Fix crash when selecting text with changes

When selecting text, in some cases a DocIterator could be forwarded
to a (non-existant) paragraph after the end. The critical part of
this fix is to break the loop at the correct place. The following
are additional improvements:

- increase readability by defining a bool named "in_last_par"
- use cur.selectionEnd().pit() instead of cur.selectionEnd().paragraph().id()
- use it.lastpos() instead of it.paragraph().size()

This commit fixes a regression introduced by 23de5e5e, and reported
at #11204.

Thanks to Jürgen and JMarc.

(cherry picked from commit d12798759aeb7bae77bec63098fd81f8cc9e554b)