John Mayer got a taste of the big, bad Apple yesterday hanging out in a Lower East Side bar — as a fellow male patron was stabbed in the stomach. The ladies’-man rocker, clad in a leather jacket and jeans, was seen fleeing GoldBar in the wee hours with a gaggle of patrons from the hip lounge on Broome Street after the bloodshed. The 28-year-old victim, who had gotten into an argu ment with his attacker, was wheeled out on a stretcher moments later and treated for non-life-threatening injuries.