cgraph: squash some -Wconversion warnings

cgraph: squash some -Wconversion warnings

This squashes the following warnings on CentOS 7 where the compiler believes
the sequence ID may overflow:

  node.c: In function 'agnodebefore':
  node.c:370:23: warning: conversion to 'unsigned int:28' from 'int' may alter
    its value [-Wconversion]
     AGSEQ(n) = AGSEQ(n) + 1;
                         ^
  node.c:376:26: warning: conversion to 'unsigned int:28' from 'int' may alter
    its value [-Wconversion]
    AGSEQ(snd) = AGSEQ(fst) - 1;
                            ^