Unless you bought a dodgy laptop and charger or it was built by a noob, it should have been through testing and regulations.
Buy a smoke alarm and put it right next to your laptop if you are worried. And make sure you have adequate ventilation around the transformer and laptop. I agree with the others here, I have all sorts of electricals on 24/7 (Laptop, Desktop, TV, DVD, Sky box, phone chargers etc) and have never had a problem.
I fly model helicopters which used high power lipo batteries, which are very flamable if damaged in a crash. I am recomended to charge them on metal tray to help reduce fire risks. You could improvise on that idea if you are worried.