No, masturbation does not stunt one's growth. Of course, for other reasons, you don't want to be obsessive about it.
Masturbating at your age is okay. You are perfectly normal. you have still few years to grow. Keep your self happy and do not worry about growth.you will grow normally.
No, masturbation (no matter how often) cannot in any way or fashion affect one's height or growth. Barring disease, a person's height is normally determined by inherited genes and, likely, quality of nutrition during the growth years. Judging by the figures you give, genes are the dominant decider---but there are always exceptions, so you may well gain (or by now have gained) more height. You are still, at 18, in possible growth period.