Bluetooth: ISO: Fix UAF on iso_sock_timeout

conn->sk maybe have been unlinked/freed while waiting for iso_conn_lock
so this checks if the conn->sk is still valid by checking if it part of
iso_sk_list.

Fixes: ccf74f23 ("Bluetooth: Add BTPROTO_ISO socket type")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>