IrDA: always initialize buffer before accessing it

Bug: 10716
Change-Id: I8e54f750b70076758d5fd0d7f35f9c782f5d0bf9
Reviewed-on: https://code.wireshark.org/review/5398
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>