dlist_check: add check for nodes pointing to themselves

Hi hackers,

I accidentally called the function dlist_push_head consecutively on the same node,
inserting it into a doubly linked list, which caused an infinite loop.
After adding the compile parameter -DILIST_DEBUG, an infinite loop occurred in the dlist_check function,
revealing that the next pointer of the current node was pointing to itself.
I am attempting to fix this issue with the following patch.

regards,
Lei Yang