If a weak ref callback causes disposal of a v8object associated with
a qobject, the later qqmldata::destroyed() handler could cause a
double dispose, due to 753d9f4be5.
Change-Id: I07c1c8e2e7b444a7e873da26bc4d0c19bcfe57b5
Reviewed-by: Matthew Vogt <matthew.vogt@nokia.com>