0002434: neato -n2 crashes on certain input

This is about http://graphviz.org/mantisbt/view.php?id=2434

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 at least give a warning,
rather than crash completely.

The warning (and suppressing the crash) is in this branch. Not sure about the root cause is though.

The reason for the crash is basically that no object is appended (and there is also no room allocated for that object) but the object pointer is increased.
`HAVE_EDGE(ed)` should probably be true at this point but somehow isn't.