Add a check for the zero vector in `Vector.normalize()`
Add a check for the zero vector in `Vector.normalize()`

This fixes coloring of lines!

In `Isomer.prototype_addPath` we find the normal of the path
and use it to determine the color. However, the normal of a
*line* returns the zero vector because math, and we probably
want to check for the zero vector when dividing by the
magnitude in `Vector.normalize()`.

sh0ber/isomer,WahlbergRu/Tank-fortress,mcanthony/isomer,mcanthony/isomer,WahlbergRu/Tank-fortress,jdan/isomer,jdan/isomer,sh0ber/isomer