Clean up overly paranoid checks in mac8.c

Clean up overly paranoid checks in mac8.c

Andres' compiler points out, quite correctly, that there's no need for
some of the overly paranoid checks which were put into mac8.c.  Remove
those, as they're useless, add some comments and make a few other minor
improvements- reduce the size of hexlookup by making it a char array
instead of an int array, and pass in the ptr location directly instead
of making hex2_to_uchar re-calculate the location based off the offset
every time.