qemu-img: Avoid setting ret to unused value in img_convert()

Coverity points out that we assign the return value from
bdrv_snapshot_load_tmp() to 'ret' in img_convert(), but then
never use that variable. (We check for failure by looking
at local_err instead.) Drop the unused assignment, bringing
the call into line with the following call to
bdrv_snapshot_laod_tmp_by_id_or_name().

(Fixes CID 1247240.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1486744104-15590-3-git-send-email-peter.maydell@linaro.org
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>