Fix for Bug 10240.
Include padding length in calculation of correct attribute length

Change-Id: I569c6a9e1be39e3bb997e797a094b80cdcba6b07
Reviewed-on: https://code.wireshark.org/review/2747
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>