Use asserts in dbl_list

codegen/dbl_list/snippets/list_remove

@@ -1,7 +1,6 @@
+  assert(list != NULL);
+
   {NodeType} *output = NULL;
-  if (!list) {{
-    goto RETURN_{Tupper}_LIST_REMOVE;
-  }}
 
   if (index == 0) {{
     output = wapp_{Tlower}_list_pop_front(list);