gtkdnd: Account for setting a same icon helper
gtkdnd: Account for setting a same icon helper

g_set_object() will take care of ref'ing before destroying the previous
instance, which might actually be the same pointer.

https://bugzilla.gnome.org/show_bug.cgi?id=751401

jadahl/gtk,chergert/gtk,Adamovskiy/gtk,grubersjoe/adwaita,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,davidgumberg/gtk,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk