Fix v.getColormap TC_ skin number values not applying correctly on replays.

Fixes #200 (closed).

v.getColormap has a check if we're running on a replay, and if it is, use a saved skin colormap instead. Though this does not check if the skinnum value itself is a TC_ value, and therefore will result in them not working in replays.

This merge request just adds a check if skinnum is actually positive beforehand, though once again I'm welcome for a better fix.