Implement avl_ndel()

This PR completes PR #145.
With `printf` and other va_arg functions, you can spam the stack as much as you want. The function will not care as it only takes information about the arguments from the formatting string.
That's why they cause so many security issues in some programs.
The only thing which could prevent compilation are compiler warnings.
Frankly, if there is no way to print a message without any argument, the macro is buggy.