If the caller provides a file name, these function load the image, set
it as the wallpaper and make sure that the memory used for the image
data is freed immediately. However, this was also done when the caller
provided a cairo surface, thus breaking their surface.
Fix the code so that it does not finish surfaces that it did not create
itself.
Fixes: https://github.com/awesomeWM/awesome/issues/1570
Signed-off-by: Uli Schlachter <psychon@znc.in>
4fc396ec57