The perils of a sky-high balcony, not unknown in glass towered Toronto, accompanied the death of a bright young advertising executive in Manhattan Wednesday. Jennifer Rosoff, 35, was smoking a ciigarette on the balcony of her on East 57 St. apartment with a man she had apparently invited home. The woman casually stretched a leg out and propped her foot on a metal balcony railing at the corner of her balcony. The man later told police he offered the woman a warning that her position might be dangerous. Rosoff told him she did it all the time. A moment later the railing collapsed into a V-shaped killer that catapulted the woman off the balcony and sent her 17 floors to her death. She landed on first floor scaffolding. Fire and police found her male friend screaming and dazed on the street below.