Dont display interior of glasslike_framed node when its not defined.

Fixes https://github.com/minetest/minetest/issues/1514

src/content_mapblock.cpp

@@ -946,7 +946,7 @@ void mapblock_mesh_generate_special(MeshMakeData *data,
 				makeCuboid(&collector, box, &glass_tiles[i], 1, c, txc2);
 			}
 
-			if (param2 > 0){
+			if (param2 > 0 && f.special_tiles[0].texture) {
 				// Interior volume level is in range 0 .. 63,
 				// convert it to -0.5 .. 0.5
 				float vlev = (((float)param2 / 63.0 ) * 2.0 - 1.0);