mih: Fix some bugs; Add an XXX comment.

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

Change-Id: I5bd43b34b08c1fdbf1cbc2adb5e069a6a796e094
Reviewed-on: https://code.wireshark.org/review/5129
Reviewed-by: Bill Meier <wmeier@newsguy.com>