Limit light_source in the engine (#4814)

Since light_source>15 causes crash, it must be limited.

src/voxelalgorithms.cpp

@@ -264,6 +264,7 @@ struct LightQueue {
 		const mapblock_v3 &block_pos, MapBlock *block,
 		direction source_dir)
 	{
+		assert(light <= LIGHT_SUN);
 		lights[light].push_back(
 			ChangingLight(rel_pos, block_pos, block, source_dir));
 	}