Bluetooth: btusb: validate Realtek vendor event length

Bluetooth: btusb: validate Realtek vendor event length

btusb_recv_event_realtek() 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.

Fixes: 044014ce85a1 ("Bluetooth: btrtl: Add Realtek devcoredump support")
Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>