Workaround to fix crash reported in #2209

src/admesh/normals.cpp

@@ -108,6 +108,9 @@ stl_fix_normal_directions(stl_file *stl) {
 
   if (stl->error) return;
 
+  // this may happen for malformed models, see: https://github.com/prusa3d/Slic3r/issues/2209
+  if (stl->stats.number_of_facets == 0) return;
+
   /* Initialize linked list. */
   head = (struct stl_normal*)malloc(sizeof(struct stl_normal));
   if(head == NULL) perror("stl_fix_normal_directions");