Fog_ParseWorldspawn: before reading the worldspawn value, clear the fog rgb color back to the default value (0.3).

This fixes a bug seen in jam3_tronyn.bsp (which only has a density value in the worldspawn), where the fog color of the last map played would carry over.

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk/quakespasm@1140 af15c1b1-3010-417e-b628-4374ebc0bcbd