stream: Traverse graph after modification

stream: Traverse graph after modification

bdrv_cor_filter_drop() modifies the block graph.  That means that other
parties can also modify the block graph before it returns.  Therefore,
we cannot assume that the result of a graph traversal we did before
remains valid afterwards.

We should thus fetch `base` and `unfiltered_base` afterwards instead of
before.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211111120829.81329-2-hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211115145409.176785-2-kwolf@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>