Bluetooth: hci_event: Ignore multiple conn complete events

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: hci_event: Ignore multiple conn complete events When one of the three connection complete events is received multiple times for the same handle, the device is registered multiple times which leads to memory corruptions. Therefore, consequent events for a single connection are ignored. The conn->state can hold different values, therefore HCI_CONN_HANDLE_UNSET is introduced to identify new connections. To make sure the events do not contain this or another invalid handle HCI_CONN_HANDLE_MAX and checks are introduced. Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=215497