Don't attemt fullbright processing for an unavailable skin.

In the end, it turns out this is the correct fix for the gl seg on
overkill, because build_skin will correctly use the fully setup player skin
if the glskin doesn't have a texture associated with it.

libs/models/gl_skin.c

@@ -245,6 +245,7 @@ gl_Skin_SetupSkin (skin_t *skin, int cmap)
 		}
 		return;
 	}
+	if (s->tex)
 		do_fb_skin (s);
 	build_skin (skin, cmap);
 }