postproc: fix crash on unpositioned edge

There seem to be cases where edges don't get a position in the layout.
Not sure where this bug is exactly, but we should give a warning,
rather than crash completely.
0002434: neatogen: fix spline not being created

We try to make a multi spline when there are multiple edges between two
nodes. We only make create this multi spline for the leader of these
edges.
If the leader already has a spline set and we are supposed to use given
splines (Nop > 1), then no multi spline will be created at all,
also not for other splines.

So we don't include these splines in the search for equivalent edges.
Merge branch 'bug_2434' of https://github.com/JonnyJD/graphviz
Slight change to logic and comment.