fix integer overflow in ordering function

fix integer overflow in ordering function

The test case for #1906 triggers an overflow in this function, viewable with
UBSan:

  $ dot -Kirco -Tgv -o /dev/null tests/graphs/root.gv
  /builds/graphviz/graphviz/lib/label/xlabels.c:35:15: runtime error: signed
    integer overflow: -1884993080 - 1219985688 cannot be represented in type
    'int'

Gitlab: #1906