block/backup: fix backup_cow_with_offload for last cluster

block/backup: fix backup_cow_with_offload for last cluster

We shouldn't try to copy bytes beyond EOF. Fix it.

Fixes: 9ded4a0114968e
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20190920142056.12778-3-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 1048ddf0a32dcdaa952e581bd503d49adad527cc)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>