Fix memory corruption when multiple QML engines have JavaScript wrappers for the...

Fix memory corruption when multiple QML engines have JavaScript wrappers for the same QObject

It's possible that the same QObject is exposed to multiple JavaScript
environments, for which we have this "extra" hack in the form of a QMap.  The
common case is that QQmlData has a QV4::WeakValue that points to the JS wrapper
for the object. However in the rare case of multiple exposure, a map in the
other engines stores those references. That map was erroneously storing
pointers to temporary values on the JS stack instead of heap pointers.

Change-Id: I8587f9921a9b4f9efd288326d00cebc25ad0bc12
Task-number: QTBUG-45051
Reviewed-by: Lars Knoll <lars.knoll@digia.com>