Bluetooth: controller: Prevent assert in ll_conn_iso_stream_get_by_acl

Do not assert in case of race conditions or asymmetric CIS
create/release. There are cases where ll_conn_iso_stream_get_by_group
legally returns NULL, and this should just cause for-loop continuing to
next iteration.

Signed-off-by: Morten Priess <mtpr@oticon.com>