Chris Brown has been sued again over an alleged assault at one of his shows over the weekend, this time by a security guard who said he was “severely injured” trying to protect four concert-goers who were being “brutalized” by the musician and his entourage. RadarOnline.com got an exclusive first look at the complaint filed Wednesday on behalf of Frederick Overpeck, the 58-year-old head of back-house security at Dickies Arena in Fort Worth, Texas, who became caught up in the chaos he described as a “prison yard beat down” on the night of June 20. In his complaint, Overpeck, a former correction…