Make path_recv() and poly_recv() reject paths/polygons containing no points.

Make path_recv() and poly_recv() reject paths/polygons containing no points.
The zero-point case is sensible so far as the data structure is concerned,
so maybe we ought to allow it sometime; but right now the textual input
routines for these types don't allow it, and it seems that not all the
functions for the types are prepared to cope.
Report and patch by Merlin Moncure.