v.build.polylines: fix cats=no

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@63322 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.build.polylines/walk.c

@@ -96,6 +96,7 @@ int walk_forward_and_pick_up_coords(struct Map_info *map,
     }
     else {
 	cats_tmp = NULL;
+	Vect_reset_cats(Cats);
     }
 
     Vect_reset_line(points);