block/replication.c: Avoid cancelling the job twice

block/replication.c: Avoid cancelling the job twice

If qemu in colo secondary mode is stopped, it crashes because
s->backup_job is canceled twice: First with job_cancel_sync_all()
in qemu_cleanup() and then in replication_stop().

Fix this by assigning NULL to s->backup_job when the job completes
so replication_stop() and replication_do_checkpoint() won't touch
the job.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Message-Id: <20200511090801.7ed5d8f3@luklap>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>