Make sure listener DCBs are removed cleanly from the list

As listener DCBs can be added and removed from the polling system multiple
times, the DCBs need to be reset to a clean state when they are removed.

server/core/dcb.c

@@ -3471,6 +3471,11 @@ static void dcb_remove_from_list(DCB *dcb)
         }
     }
 
+    /** Reset the next and tail pointers so that if this DCB is added to the list
+     * again, it will be in a clean state. */
+    dcb->thread.next = NULL;
+    dcb->thread.tail = NULL;
+
     spinlock_release(&all_dcbs_lock[dcb->thread.id]);
 }