Thanks to Martin Friebe for finding and submitting a fix for this bug!

A table with maximum number of key segments and maximum length key name
would have a corrupted .frm file, due to an incorrect calculation of the
complete key length.  Now the key length is computed correctly (I hope) :-)

MyISAM would reject a table with the maximum number of keys and the maximum
number of key segments in all keys.  It would allow one less than this total
maximum.  Now MyISAM accepts a table defined with the maximum.  (This is a
very minor issue.)