bug in dia_image_release()

[Dan Cohn]

Hi folks,

        dont have time to track this down right now, so someone please
pick it up. the creation of a broken image, Edit->Copy, Edit->Cut causes a
segv. (sometimes it seems to require a third Edit->Copy after these
two, but not always) Seems that this combination passes a bad DiaImage
reference (but not a null one, which is all that is being checked for)
from image_destroy() to dia_image_release(). [segv is on the
image->refcount-- line, dia.image:82]

        thanks,

        dc