R_InitExtraColormaps fix

This fixes the R_InitExtraColormaps function so that it correctly realises there are no "extra colormaps" actually in SRB2's files, rather than telling you there are 6 extra colormaps on game startup (or however many WAD files total you have added).

For those who don't know what the function does, it searches for C_START/C_END markers, and if they exist, adds any lumps inbetween to a list of extra colormaps internally. They are never used by the game though, since we last supported colormaps of that kind so long ago I don't think anyone remembers when anymore (definitely more than a decade at least).

Merging to next since I don't know if this would cause any netplay issues or not tbh.