Push a frame for thread toplevel

A new Thread should have a toplevel frame, even if the code it runs
has been frame-optimized or is not Ruby code. This prevents any
attempts to get a current binding from failing to access the frame
stack.

Fixes jruby#9308.

Author: headius

core/src/main/java/org/jruby/internal/runtime/RubyRunnable.java

@@ -100,6 +100,7 @@ public void run() {
         // uber-ThreadKill catcher, since it should always just mean "be dead"
         try {
             // Push a frame for the toplevel of the thread
+            context.pushFrame();
 
             // Call the thread's code
             Block threadBlock = proc.getBlock();