Check for a divide by zero in the step calculation

[expenses]

Hi, thanks for the great library! I'm still evaluating if it'll work for my needs but I managed to find one SEGFAULT along the way.

float l = so_length2(ad);
int iterations = (int)(l * 5.0f); // TODO: is this the best value?
float step = 1.0f / iterations;

with this code, if l is below 0.2 (0.1995 or so in my case) then the number of iterations will be 0 and step will be nan, which breaks things down the chain and ends up with an attempt to read an out of bounds value from the lightmap.