From wiki {Safety_(firearms)}
Most modern double-action revolvers have an
internal safety, either a hammer block or
a transfer bar, that positively
prevents firing without the trigger being pulled.
Yes, I know about transfer bars, I have one on my own S&W revolver. When most people talk about a “safety” they mean a control that locks the trigger from working until the safety is manually flipped or manipulated, such as the thumb safety on the 1911.
Somehow I doubt the cop was fiddling with the transfer bar when he managed his negligent discharge.