Check if mask_greatest_bit_offset is within limits.

The result of the '>>' expression is undefined if mask_greatest_bit_offset = 0.

Change-Id: I15b094a5220a5075878bad65ab0b8c5279e80e8a
Reviewed-on: https://code.wireshark.org/review/4723
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>