Fix for assert in V8 after a string is converted to a symbol

When a string is converted to a symbol by SymbolKey::AsObject, its
symbol_id is undefined.

Change-Id: I63c41d52e309f67a4c8fc2629bdc9fb6a18fd2b5
bug: https://bugreports.qt-project.org/browse/QTBUG-31513
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>