Permit list_delete_all to accept a NULL key.

src/common/list.c

@@ -385,7 +385,6 @@ list_delete_all (List l, ListFindF f, void *key)
 
     assert(l != NULL);
     assert(f != NULL);
-    assert(key != NULL);
     list_mutex_lock(&l->mutex);
     assert(l->magic == LIST_MAGIC);
     pp = &l->head;