Fix crash when dropping image.
Apparently it can happen that QMimeData::hasImage() can return true, but QMimeData::imageData returns a null QImage. I guess this should be considered a Qt bug. This fix simply introduces a check for a null image to avoid a crash when this happens. I guess we should investigate why Qt ends up in this state in the first place, but it doesn't hurt to be careful. REVIEW: 110593