text: fix critical in paste_received

Here we calculate the length only in the truncate_multiline condition. Then we pass pos - 1 to gtk_accessible_text_update_contents() as the end position, triggering this critical that checks to ensure start <= end. Fix it by always calculating the length of the string that we insert.

This is the first bug fixed as a result of enabling fatal criticals by default in Epiphany! 🎉

Fixes #6734 (closed)