Clear the skybox textures on map load. Use memset to clear skyname.

Clear the skybox textures on map load. Use memset to clear skyname.

This fixes an issue where the skybox textures of the first map you load
stick around in subsequent maps. There was a missing call to
R_SkinFrame_PurgeSkinFrame. Also use memset to clear skyname for good
measure. This may not have actually been part of the bug but it's harmless
and it wasn't clearing the entire array previously.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12527 d7cf8633-e32d-0410-b094-e92efae38249