mih: Fix some bugs; Add an XXX comment.

  One of the bugs: (2^32) is *not* 2 to the power of 32.

Change-Id: Id4dab16380f757e89c05be4182da3ac92c92abd6
Reviewed-on: https://code.wireshark.org/review/5127
Reviewed-by: Bill Meier <wmeier@newsguy.com>