MADONNA pulled a Sinead O'Connor on NBC's Saturday Night Live, but this time the photo-shredding was for sheer laughs, not shock value. After belting out Bad Girl, from her Erotica album, Madonna echoed O'Connor's words: ''Fight the real enemy!'' Then Madonna ripped up a picture of not Pope John Paul, but Joey Buttafuoco, the man in the middle of all those Amy ''Long Island Lolita'' Fisher TV movies. After Ms O'Connor ended a song by angrily tearing up a photo of the Pope on the October 3 Saturday Night Live, viewers flooded the NBC switchboard to protest. Even the Material Girl complained, saying the stubble-headed Irish singer had gone too far to make a point. ''I think there's a better way to present her ideas,'' Madonna said, ''rather than ripping up an image that means a lot to other people.''