Widen openflow variables and check for underflows

As suggested by Alexis reviewing change Ie76c5a810af927b, this issue was
scattered throughout the v4/v5 openflow dissectors. Hopefully I got all of them.
Original issue found in bug 10259.

Change-Id: Idf87deea34132aec5cd4e8f83264906a29b15568
Reviewed-on: https://code.wireshark.org/review/2878
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>