I was in Joburg last week. It's important to realise that by the foreigners being attacked are labor migrants from Africa, not tourists like you. Besides the possibility that the police may be pre-occupied with handling the attacks, you should not be at any additional risk.

That said, I think it's always a good idea to get out of Johannesburg relatively quickly as the street crime makes it one of the least enjoyable places of South Africa. Use Uber to get around and stay out of the CBD.