Bluetooth: fix null ptr deref on hci_sync_conn_complete_evt

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: fix null ptr deref on hci_sync_conn_complete_evt

This event is just specified for SCO and eSCO link types.

On the reception of a HCI_Synchronous_Connection_Complete for a BDADDR

of an existing LE connection, LE link type and a status that triggers the

second case of the packet processing a NULL pointer dereference happens,

as conn->link is NULL.