There's an autonomous flight termination system which triggers if it strays outside the planned flight corridor; any debris that survives reentry should then land in the advertised safety zone.
If it doesn't explode it might be light enough to survive reentry, after sailing on for a short while. In that case a large chunk of metal will come down either off the coast of South Africa or if continues on in its orbit potentially off the coast of Australia.
[1] has the planned flight path, as well as the impact zones.
the onboard control system and flight termination system are programmed to explode if it deviates from a specific and allowed path of trajectory/speed/functional engine thrust. The last thing anyone wants is a partially broken starship going into an uncontrolled suborbital velocity that lands on a city in Africa.
They have a bunch of explosives strapped on the rocket and can give a radio command to blow the ship up. It can even decide to explode itself if the readings go haywire.
It is called the Flight Termination System and it is very common on non-manned flights now.
It could be noted that manually operated flight termination systems have been used even on manned spaceflight, each and every space shuttle flight had a termination system under human ground control.
