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>

src/vertex-clipping.c

@@ -206,6 +206,9 @@ clip_polygon_left(struct clip_context *ctx, const struct polygon8 *src,
 	enum path_transition trans;
 	int i;
 
+	if (src->n < 2)
+		return 0;
+
 	clip_context_prepare(ctx, src, dst_x, dst_y);
 	for (i = 0; i < src->n; i++) {
 		trans = path_transition_left_edge(ctx, src->x[i], src->y[i]);
@@ -222,6 +225,9 @@ clip_polygon_right(struct clip_context *ctx, const struct polygon8 *src,
 	enum path_transition trans;
 	int i;
 
+	if (src->n < 2)
+		return 0;
+
 	clip_context_prepare(ctx, src, dst_x, dst_y);
 	for (i = 0; i < src->n; i++) {
 		trans = path_transition_right_edge(ctx, src->x[i], src->y[i]);
@@ -238,6 +244,9 @@ clip_polygon_top(struct clip_context *ctx, const struct polygon8 *src,
 	enum path_transition trans;
 	int i;
 
+	if (src->n < 2)
+		return 0;
+
 	clip_context_prepare(ctx, src, dst_x, dst_y);
 	for (i = 0; i < src->n; i++) {
 		trans = path_transition_top_edge(ctx, src->x[i], src->y[i]);
@@ -254,6 +263,9 @@ clip_polygon_bottom(struct clip_context *ctx, const struct polygon8 *src,
 	enum path_transition trans;
 	int i;
 
+	if (src->n < 2)
+		return 0;
+
 	clip_context_prepare(ctx, src, dst_x, dst_y);
 	for (i = 0; i < src->n; i++) {
 		trans = path_transition_bottom_edge(ctx, src->x[i], src->y[i]);