Fix GC errors with Error object

Don't forget to properly mark the stack value we're keeping

Change-Id: Ifb002a5e8519e40cd1a4c7f7845171452a585562
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Fix GC errors in small objects during construction

When creating small objects with for example only one accessor property,
then that is stored in the inline member allocation, which needs to
be initialized as well to prevent GCs during construction to access
uninitialized members.

Change-Id: Ie20d27a650e09475845f8ee6aa79b8e62a4bb795
Reviewed-by: Lars Knoll <lars.knoll@digia.com>