Add guards on debug output.

src/core/org/luaj/vm2/Print.java

@@ -365,9 +365,13 @@ public class Print extends Lua {
 				break;
 			case LuaValue.TUSERDATA:
 				Object o = v.touserdata();
+				if ( o != null ) {
 					String n = o.getClass().getName();
 					n = n.substring(n.lastIndexOf('.')+1);
 					ps.print( n+": "+Integer.toHexString(o.hashCode()) );
+				} else {
+					ps.print( v.toString() );
+				}
 				break;
 			default:
 				ps.print(v.tojstring());

src/core/org/luaj/vm2/lib/DebugLib.java

@@ -352,7 +352,7 @@ public class DebugLib extends VarArgFunction {
 			return;
 		DebugInfo di = ds.getDebugInfo();
 		if(TRACE)Print.printState(di.closure, pc, di.stack, top, di.varargs);		
-		ds.getDebugInfo().bytecode( pc, extras, top );
+		di.bytecode( pc, extras, top );
 		if ( ds.hookcount > 0 ) {
 			if ( ++ds.hookcodes >= ds.hookcount ) {
 				ds.hookcodes = 0;