If size of a flash image file is 0 ubiformat silently formats the volume.
Check size and abort if size == 0. This may be the case if an empty or tftp
mounted file is used.
Signed-off-by: Jan Weitzel <j.weitzel@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>