BUG/MEDIUM: mt_lists: Make sure we set the deleted element to NULL;

In MT_LIST_DEL_SAFE(), when the code was changed to use a temporary variable
instead of using the provided pointer directly, we shouldn't have changed
the code that set the pointer to NULL, as we really want the pointer
provided to be nullified, otherwise other parts of the code won't know
we just deleted an element, and bad things will happen.

This should be backported to 2.1.

include/common/mini-clist.h

@@ -620,7 +620,7 @@ struct cond_wordlist {
 		struct mt_list *el = (_el);                                \
 		(el)->prev = (el);                                         \
 		(el)->next = (el);                                         \
-		(el) = NULL;                                               \
+		(_el) = NULL;                                              \
 	} while (0)
 
 /* Simpler FOREACH_ITEM_SAFE macro inspired from Linux sources.