Tracking: Early output when drawing 0 path points

The rumors says that immBegin() will assert fail on 0 count. Not sure where exactly
that happens (can't reproduce that), but early output can't hurt.

source/blender/editors/space_clip/clip_draw.c

@@ -462,6 +462,9 @@ static void draw_track_path_points(const TrackPathPoint *path,
                                    const int start_point,
                                    const int num_points)
 {
+  if (num_points == 0) {
+    return;
+  }
   immBegin(GPU_PRIM_POINTS, num_points);
   for (int i = 0; i < num_points; i++) {
     const TrackPathPoint *point = &path[i + start_point];