masks: avoid assertion failure in early phase of path generation

src/develop/masks/path.c

@@ -342,6 +342,8 @@ static void _path_points_recurs(float *p1, float *p2, double tmin, double tmax,
 /** find all self intersections in a path */
 static int _path_find_self_intersection(dt_masks_dynbuf_t *inter, int nb_corners, float *border, int border_count)
 {
+  if(nb_corners == 0 || border_count == 0) return 0;
+
   int inter_count = 0;
 
   // we search extreme points in x and y
@@ -526,7 +528,7 @@ static int _path_get_points_border(dt_develop_t *dev, dt_masks_form_t *form, int
     }
   }
 
-  intersections = dt_masks_dynbuf_init(10 * nb, "path intersections");
+  intersections = dt_masks_dynbuf_init(10 * MAX(nb, 1), "path intersections");
   if(intersections == NULL)
   {
     dt_masks_dynbuf_free(dpoints);