Cleanup Fiber in disposeThread()

* So this runs even if there was an exception during the poll()
  that PolyglotThread does when entering the context.

Author: eregon

src/main/java/org/truffleruby/RubyLanguage.java

@@ -668,7 +668,8 @@ public void disposeThread(RubyContext context, Thread thread) {
                 }
                 context.getThreadManager().cleanupThreadState(rubyThread, thread);
             } else { // (non-root) Fiber
-                // Fibers are always cleaned up by their thread's cleanup with FiberManager#killOtherFibers()
+                var fiber = this.rubyFiber.get(thread);
+                context.fiberManager.cleanup(fiber, thread);
             }
             return;
         }

src/main/java/org/truffleruby/core/fiber/FiberManager.java

@@ -182,9 +182,6 @@ private void fiberMain(RubyContext context, RubyFiber fiber, RubyProc block, Nod
     }
 
     private void afterLeave(RubyFiber fiber) {
-        final Thread thread = Thread.currentThread();
-        cleanup(fiber, thread);
-
         if (fiber.lastMessage != null) {
             addToMessageQueue(fiber.returnFiber, fiber.lastMessage);
             fiber.returnFiber = null;