clear Thread::javaThread before entering zombie state, since clearing it in Thread::dispose is too late - the reference may already be invalid since we don't visit GC roots for zombie threads

src/jnienv.cpp

@@ -53,6 +53,9 @@ DetachCurrentThread(Machine* m)
     ACQUIRE_RAW(t, t->m->stateLock);
 
     enter(t, Thread::ActiveState);
+
+    threadPeer(t, t->javaThread) = 0;
+
     enter(t, Thread::ZombieState);
 
     t->state = Thread::JoinedState;

src/machine.cpp

@@ -2201,6 +2201,7 @@ Thread::exit()
     if (m->liveCount == 1) {
       turnOffTheLights(this);
     } else {
+      threadPeer(this, javaThread) = 0;
       enter(this, Thread::ZombieState);
     }
   }
@@ -2209,8 +2210,6 @@ Thread::exit()
 void
 Thread::dispose()
 {
-  threadPeer(this, javaThread) = 0;
-
   if (systemThread) {
     systemThread->dispose();
   }