Bluetooth: hci_conn: return ERR_PTR instead of NULL when there is no link

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: hci_conn: return ERR_PTR instead of NULL when there is no link hci_connect_sco currently returns NULL when there is no link (i.e. when hci_conn_link() returns NULL). sco_connect() expects an ERR_PTR in case of any error (see line 266 in sco.c). Thus, hcon set as NULL passes through to sco_conn_add(), which tries to get hcon->hdev, resulting in dereferencing a NULL pointer as reported by syzkaller. The same issue exists for iso_connect_cis() calling hci_connect_cis(). Thus, make hci_connect_sco() and hci_connect_cis() return ERR_PTR instead of NULL.