It could very well be that there’s something amiss in the car’s ignition barrel (which lives on the steering column, remember) or its wiring and connectors that is causing the power to the ignition to be cut when you turn the wheel. The problem could also be a short-circuit somewhere in the wiring in the column (and there’s plenty of it) that is causing the problem. Either way, you need to get it fixed immediately, as having a car cut out when you least expect it is can lead to all sorts of extremely dangerous situations. Another possibility is that there’s something wrong with the car’s power steering system which is overloading at the point you turn the wheel hard and causing the engine to stall. Again, it needs to be fixed now and I wouldn’t be driving the car anywhere other than to a workshop for diagnosis.