The avl_destroy() function should not get NULLsfg
Warn if the result of avl_alloc() is not used
Ensure that the ptr args of avl_add() are not NULL
Ensure that the ptr args of avl_del() are not NULL
Ensure that the ptr args of avl_ndel() are not NULL
Ensure that the ptr args of avl_find() are not NULL and the result gets used
The avl_get_hash() function is deprecated, as the user should not handle avl elements on his own