Window: fix buffer_coord when a line buffer range is empty
Window: fix buffer_coord when a line buffer range is empty

Fixes #1711

jkonecny12/kakoune,lenormf/kakoune,lenormf/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,Somasis/kakoune,jjthrash/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,danr/kakoune,mawww/kakoune,jjthrash/kakoune,lenormf/kakoune,danr/kakoune,alexherbo2/kakoune,Somasis/kakoune,jjthrash/kakoune,mawww/kakoune,danr/kakoune,mawww/kakoune,jjthrash/kakoune,mawww/kakoune,alexherbo2/kakoune,occivink/kakoune,casimir/kakoune,occivink/kakoune,occivink/kakoune,danr/kakoune,occivink/kakoune,casimir/kakoune,Somasis/kakoune,lenormf/kakoune,Somasis/kakoune,casimir/kakoune,jkonecny12/kakoune,casimir/kakoune