Patton Oswalt returns as Agent Koenig with a special message to greet San Diego Comic-Con attendees in preparation for the return of "Marvel's Agents of S.H.I.E.L.D." on September 23 at 9:00 p.m. ET on ABC! Subscribe to Marvel: http://bit.ly/WeO3YJ Keep up with Marvel on: TWITTER: https://twitter.com/marvel FACEBOOK: http://www.facebook.com/Marvel TUMBLR: http://marvelentertainment.tumblr.com/…