copy() may result in a failing malloc and thus return a QImage with a
null d. Hence the need for a null check before incrementing detach_no.
Task-number: QTBUG-41657
Change-Id: I868c37ef355fceaa1e2fc5f8a472e3dcc84dcadd
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>