Add missing null check when detaching a QImage

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>

src/gui/image/qimage.cpp

@@ -1081,7 +1081,8 @@ void QImage::detach()
         if (d->ref.load() != 1 || d->ro_data)
             *this = copy();
 
-        ++d->detach_no;
+        if (d)
+            ++d->detach_no;
     }
 }