disable post_class_unload() for non JavaThread initiators

The current post_class_unload() function can crash the
VM when a non JavaThread initiates a class unload event.
It is also possible that the non JavaThread can be
corrupted instead of crashing the VM.

Until JDK-8173658 can be fixed for non JavaThread
initiators of a class unload event, the function should
disabled.

$ hg diff src/share/vm//prims/jvmtiExport.cpp
diff -r 28e800db5a8e src/share/vm/prims/jvmtiExport.cpp
--- a/src/share/vm/prims/jvmtiExport.cpp Sat Jan 28 14:10:02 2017 -0700
+++ b/src/share/vm/prims/jvmtiExport.cpp Mon Jan 30 16:31:27 2017 -0700
@@ -1285,8 +1285,12 @@
     assert(thread->is_VM_thread(), "wrong thread");
 
     // get JavaThread for whom we are proxy
- JavaThread *real_thread =
- (JavaThread *)((VMThread *)thread)->vm_operation()->calling_thread();
+ Thread *calling_thread = ((VMThread *)thread)->vm_operation()->calling_thread();
+ if (!calling_thread->is_Java_thread()) {
+ // cannot post an event to a non-JavaThread
+ return;
+ }
+ JavaThread *real_thread = (JavaThread *)calling_thread;
 
     JvmtiEnvIterator it;
     for (JvmtiEnv* env = it.first(); env != NULL; env = it.next(env)) {