Ensure that we don't attempt to dispose handle twice

Ensure that we don't attempt to dispose handle twice

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 753d9f4be5960be8b11ad067b29fc87c168ee663.

Change-Id: I07c1c8e2e7b444a7e873da26bc4d0c19bcfe57b5
Reviewed-by: Matthew Vogt <matthew.vogt@nokia.com>