Fix IndexError: index out of range

[lochotzke]

Since xy is 1+size long the highest valid index is 1+(size-1) which equals size.

[cslashm]

In fact, after re-reading P1363-2000, my encoding is not correct. (Don't ask where I read the tody encoding....).
Correct one shall be:

04 X Y
02 X       if Y is even
03 X       if Y is odd

[cslashm]

fixed in incoming release