Prevent potential divide by zero in vec3_unit()

src/vec3.c

@@ -47,5 +47,6 @@ double vec3_len(vec3_t v)
 
 vec3_t vec3_unit(vec3_t v)
 {
-	return vec3_scale(v, 1.0 / vec3_len(v));
+	const double l = vec3_len(v);
+	return l == 0 ? v : vec3_scale(v, 1.0 / l);
 }