Box2d: Fixed issue #1381. Missing fixtureB in Contact when Body is removed.

Hi! I've got a simple solution to the bug #1381. :P
In the World class the destroyBody() method frees Java objects in the wrong order. All joints, fixtures and the body are removed before jniDestroyBody() method is called and usually this is a correct behaviour. But, in this case the ContactListener callbacks are invoked while destroying the body, so Java-side objects should be preserved until their native counterparts are destroyed.