stream: fix crash in stream_start() when block_job_create() fails

The code that tries to reopen a BlockDriverState in stream_start()
when the creation of a new block job fails crashes because it attempts
to dereference a pointer that is known to be NULL.

This is a regression introduced in a170a91fd3eab6155da39e740381867e,
likely because the code was copied from stream_complete().

Cc: qemu-stable@nongnu.org
Reported-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Alberto Garcia <berto@igalia.com>
Tested-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
