TracePoint thread_begin and thread_end events are broken

[headius]

The thread_begin and thread_end events used by TracePoint (2.0 feature) do not work properly because they expect execution frames to be available on ThreadContext. For example:

 $ jruby --debug -e "t = TracePoint.new(:thread_begin) {|*a| p a }; t.enable; Thread.new { }.join"
Exception in thread "RubyThread-2: -e:1" java.lang.ArrayIndexOutOfBoundsException: -1
    at org.jruby.runtime.ThreadContext.popFrame(ThreadContext.java:428)
    at org.jruby.runtime.ThreadContext.postTrace(ThreadContext.java:1184)
    at org.jruby.ext.tracepoint.TracePoint$2.eventHandler(TracePoint.java:105)
    at org.jruby.runtime.EventHook.event(EventHook.java:30)
    at org.jruby.Ruby.callEventHooks(Ruby.java:2899)
    at org.jruby.runtime.ThreadContext.trace(ThreadContext.java:648)
    at org.jruby.runtime.ThreadContext.trace(ThreadContext.java:644)
    at org.jruby.internal.runtime.RubyRunnable.run(RubyRunnable.java:98)
    at java.lang.Thread.run(Thread.java:744)

I'm not exactly sure how to properly fire these trace events since they need to happen outside the thread's block, but at that point not all ThreadContext structures necessary for execution have been prepared.

Note also that it fails on the postTrace side, which is strange.

This was discovered while fixing #1318.

[enebo]

Drive-by test and we appear to be sending tracepoints for threads now. The output is not quite the same as MRI but they seem to be happening:

jruby --debug -e 't2 = TracePoint.new(:thread_begin) { |a| puts "START: #{a}"}; t = TracePoint.new(:thread_end) {|a| puts "END: #{a}" }; t2.enable; t.enable; 10.times { Thread.new { } }; sleep 4'

Any other changes needed for output or missing state can become a new issue.