address issue where when all 256 symbols are used then num_active mus…

[mdejong]

…t be special cases to 256 instead of zero, fix int representation so that negative index value is generated when more than 127 symbols are used

[mdejong]

Here is a script that generates a file that contains all 256 symbols, this can be used to test the patch.

cat write_bytes.tcl
set outfd [open "all_bytes.bin" "w"]
fconfigure $outfd -encoding binary -translation binary

for {set i 0} {$i < 256} {incr i} {
set c [binary format c $i]
puts -nonewline $outfd $c
puts "wrote byte value $i"
}

puts "wrote all bytes"