Bluetooth: HCI_EVT fix fuzz failture

Fix possible "Conditional jump or move depends on uninitialised value"

Bug: 10396
Change-Id: I2c57f8310f88c68ae921d7e5bd1c060d0df27b8e
Reviewed-on: https://code.wireshark.org/review/3769
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>