vmdk: Clean up bdrv_open_child() return value check

bdrv_open_child() may return NULL.
Usually return value is checked for this function.
Check for return value is more reliable.

Fixes: 24bc15d1 ("vmdk: Use BdrvChild instead of BDS for references to extents")

Signed-off-by: Dmitry Frolov <frolov@swemel.ru>
Message-ID: <20230831125926.796205-1-frolov@swemel.ru>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>