Part way through the description of the file structure, the document says:

These values serve as indices into an array of ttinfo
structures that appears next in the file; these structures
are defined as follows:

It isn't immediately clear that there are tzh_typecnt entries in the array of ttinfo structures.

Maybe the modification is as simple as:

These values serve as indices into an array of ttinfo
structures (with tzh_typecnt entries) that appears next in the file; these structures
are defined as follows:

--
Jonathan Leffler < jonathan.leffler@gmail.com>  #include <disclaimer.h>
Guardian of DBD::Informix - v2007.0914 - http://dbi.perl.org
"Blessed are we who can laugh at ourselves, for we shall never cease to be amused."