In elm_image, efl_file_get() was called on a legacy Evas Image
object, which generated an ERR message. This was way too much
noise for something that actually works.
It's best not to call efl_file_get/set API on a legacy
object, but it can be convenient for smooth code transitions
from legacy to pure eo.
Also, add safety to those APIs.
Test case: elm_test genlist dnd, start dragging anything.