Don't crash when marking a single message: Closes #5509.
Don't crash when marking a single message: Closes #5509.

AtticHacker/geary,rschroll/geary,jimbonator/geary,robol/geary,robol/geary,emersion/geary,AtticHacker/geary,tshikaboom/geary,glebihan/geary,didrocks/geary,aatishnn/geary,orlandosanchez/geary,githubnemo/geary,mjog/geary,emersion/geary,mpdeimos/geary,emersion/geary,martinra/geary,githubnemo/geary,martinra/geary,dominickm/Geary,MohamedIM/geary-unofficial,aatishnn/geary,githubnemo/geary,tshikaboom/geary,mjog/geary,jimbonator/geary,kyoushuu/geary,dominickm/Geary,martinra/geary,mjog/geary,AtticHacker/geary,jimbonator/geary,glebihan/geary,geekvigilante/geary,kyoushuu/geary,orlandosanchez/geary,orlandosanchez/geary,tshikaboom/geary,glebihan/geary,aatishnn/geary,kpiwko/geary,rschroll/geary,abn/geary,robol/geary