Don't underrun the vertex array of empty polygons

Don't underrun the vertex array of empty polygons

This silences the following warning:
  src/vertex-clipping.c:196:22: warning: array subscript is below array
  bounds [-Warray-bounds]
    ctx->prev.x = src->x[src->n - 1];

[Pekka Paalanen: the src->n < 2 comparison comes from the fact that a
polygon with 0 or 1 points is not a polygon. A polygon with 2 points is
still degenerate, but at least it has two edges that can be clipped.]

Signed-off-by: Ondřej Majerech <oxyd.oxyd@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>