When adding a comment, return a 404 if talk doesn't exist
When adding a comment, return a 404 if talk doesn't exist

This is better than trying to access the $talk array later when it's
not an array, but boolean false...

railto/joindin-api,dstockto/joindin-api,lornajane/joindin-api,railto/joindin-api,joindin/joindin-api,dstockto/joindin-api,heiglandreas/joindin-api,lornajane/joindin-api,joindin/joindin-api,phoenixrises/joindin-api,akrabat/joindin-api,akrabat/joindin-api,phoenixrises/joindin-api,asgrim/joindin-api,asgrim/joindin-api,phoenixrises/joindin-api,asgrim/joindin-api,joindin/joindin-api,akrabat/joindin-api,heiglandreas/joindin-api,dstockto/joindin-api,railto/joindin-api,lornajane/joindin-api,heiglandreas/joindin-api