set_config: free device property value only when actually set. initialize it to 0 to avoid double frees
set_config: free device property value only when actually set.
initialize it to 0 to avoid double frees

fixes https://github.com/gphoto/libgphoto2/issues/167

gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2