6756774: fstdebug jvm fails with assertion (result != deleted_handle(),"Used a...

6756774: fstdebug jvm fails with assertion (result != deleted_handle(),"Used a deleted global handle.")
Reviewed-by: art, anthony

src/windows/native/sun/windows/awt_MenuItem.cpp

@@ -114,6 +114,7 @@ void AwtMenuItem::Dispose()
     if (m_peerObject != NULL) {
         JNI_SET_PDATA(m_peerObject, NULL);
         env->DeleteGlobalRef(m_peerObject);
+        m_peerObject = NULL;
     }
 
     AwtObject::Dispose();