MINOR: mt_lists: Appease gcc.

gcc is confused, and think p may end up being NULL in _MT_LIST_RELINK_DELETED.
It should never happen, so let gcc know that.
diff --git a/include/common/mini-clist.h b/include/common/mini-clist.h
index f18e3f0..a7b5a5d 100644
--- a/include/common/mini-clist.h
+++ b/include/common/mini-clist.h
@@ -608,6 +608,7 @@
 #define _MT_LIST_RELINK_DELETED(elt2)                                      \
     do {                                                                   \
 	    struct mt_list *n = elt2.next, *p = elt2.prev;                 \
+	    ALREADY_CHECKED(p);                                            \
 	    n->prev = p;                                                   \
 	    p->next = n;                                                   \
     } while (0);