cmd: ubi: check 'updating' before calling ubi_more_update_data()

If 0 is passed to the 'bytes' parameter in the ubi_start_update(),
there is no need to call the ubi_more_update_data(). Otherwise,
there will be a double-free of 'vol->upd_buf'.

Also check that the ubi_start_update() was called before calling
the ubi_more_update_data().

Signed-off-by: Martin Kurbanov <mmkurbanov@salutedevices.com>