Bluetooth: ISO: Fix issue with BIS tx_complete

BIS termination as broadcaster is handled different
than ACL and CIS, and in rare chances the
tx_complete for BIS may not have been completed in
the system workqueue before iso_new was called for the
same bt_conn struct (e.g. via bt_iso_cig_create), which
would perform

k_work_init(&conn->tx_complete_work, tx_complete_work);

but where conn->tx_complete_work still existed in
the system workqueue, which would cause the list
of pending items on the system workqueue to be removed
as the `next` pointer would be NULL.

This also adds an assert in bt_conn_new to prevent this
issue from appearing again.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>