CVE-2026-68339: Bluetooth: btusb: validate Realtek vendor event length

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

Bluetooth: btusb: validate Realtek vendor event length

btusbrecveventrealtek() reads the event code at data[0] and the Realtek subevent code at data[2] before deciding whether to consume a vendor event as a coredump.

For example, the two-byte event ff 00 contains a complete vendor-event header declaring zero parameters. The old classifier still reads a nonexistent third byte and can misclassify the event as a coredump if the adjacent byte is 0x34.

Require the HCI event header and first parameter to be present before inspecting the Realtek subevent code. Short events continue through the normal HCI receive path, which owns their protocol validation.