Fix yet another openflow underflow issue.

I thought I'd found all of these but I guess I hadn't; good thing the fuzz-bot
kept looking.

Bug:10314
Change-Id: I2cc209a6c87781d10cae28f2cb91400d759f5091
Reviewed-on: https://code.wireshark.org/review/3205
Reviewed-by: Evan Huus <eapache@gmail.com>