How does one enter coordinates on that device? I'd guess that as an offset from your location ("200m north of my position") might be the easiest way. If that's the case, then starting with a zero offset would be the logical choice.
You'd hope they would add some sort of safety to prevent this sort of thing, though.
It could actually be very simple. At power on the coordinates are zeroed, but "fire" is not allowed until coordinates are adjusted again. This prevents the original situation, but allows a tactical decision like "fire on own position" to be executed with at least as much speed as any other targeting decision. Firing at power on is an implicitly optimized UX for firing at the default position, and there is no reason for any target to be favored in this case.