Fix memory leak in DualThreadStack
Fix memory leak in DualThreadStack

Fixes #2556

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