Failure when reapplying a probe for JDK 15

[jbachorik]

When BTrace is using invoke dynamic (indy) (instead of generating private static handler method which is frowned upon in JDK 14 and forbidden in JDK 15) the cushion methods (empty-bodied handler methods corresponding to removed probes) do not play nice with the Indy and cause the following error:

java.lang.UnsupportedOperationException: class redefinition failed: attempted to add a method
        at java.instrument/sun.instrument.InstrumentationImpl.retransformClasses0(Native Method)
        at java.instrument/sun.instrument.InstrumentationImpl.retransformClasses(InstrumentationImpl.java:167)
        at org.openjdk.btrace.agent.Client.retransformLoaded(Client.java:500)
        at org.openjdk.btrace.agent.Main$5.run(Main.java:878)
        at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
        at java.base/java.lang.Thread.run(Thread.java:832)