Bluetooth: ISO: Fix possible UAF on iso_conn_free

This attempt to fix similar issue to sco_conn_free where if the
conn->sk is not set to NULL may lead to UAF on iso_conn_free.

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