xmlparse.c: Fix XML_Size/XML_Index cast mixup

xmlparse.c: Fix XML_Size/XML_Index cast mixup

The "MAX = (type)-1" hack only works for unsigned types:
XML_Size is unsigned but XML_Index is not.
As the positive maximum of signed integers is about
half as big as that of their unsigned counterpart,
we divide by 2.

Example for 2 bit integers:
* signed: -2, -1, 0, 1 == 2^1-1
* unsigned: 0, 1, 2, 3 == 2^2-1

Fixing 4be2cb5afcc018d996f34bbbce6374b7befad47f