qemu-img: add check for zero-length job len

The mirror job doesn't update its total length until
it has already started running, so we should translate
a zero-length job-len as meaning 0%.

Otherwise, we may get divide-by-zero faults.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -656,7 +656,8 @@ static void run_block_job(BlockJob *job, Error **errp)
 
     do {
         aio_poll(aio_context, true);
-        qemu_progress_print((float)job->offset / job->len * 100.f, 0);
+        qemu_progress_print(job->len ?
+                            ((float)job->offset / job->len * 100.f) : 0.0f, 0);
     } while (!job->ready);
 
     block_job_complete_sync(job, errp);