I think the way to treat 0/0 is to think about how you got to it in the first place.

For example, if you got a slop of 0/0 then x1-x2 must be 0 and y1 - y2 must be zero. The only way that could happen is if x1 = x2 and y1 = y2, so a slope of 0/0 means you used the same point to find slope twice.