Sanitize the element when dequing

Hi,

Here is a patch to sanitize the element in the deque operation. Your
suggestions are appreciated.

Haohui

Index: src/api/v4/queueing.h
===================================================================
--- src/api/v4/queueing.h       (revision 1696)
+++ src/api/v4/queueing.h       (working copy)
@@ -75,6 +75,7 @@
            head = tcb->list.next;                      \
        (tcb->list.next)->list.prev = tcb->list.prev;   \
        (tcb->list.prev)->list.next = tcb->list.next;   \
+    tcb->list.next = tcb->list.prev = NULL;            \
     }                                                  \
 } while(0)