neatogen: fix miscalculation of intermediate edge resources

neatogen: fix miscalculation of intermediate edge resources

`genroute` was allocating an array for edge computation upfront. But what it was
not accounting for was that some of the functions it later calls _change_ the
`pn` value it used to determine how many array elements it should allocate.
Specifically, `Pshortestpath` can add new points to the polygon, thereby causing
the walk of the (now too short) array to write out of bounds.

Gitlab: fixes #42
Reported-by: mattjj