The slang seems to be the problem. 
くんな → 来んな → 来るな
This is relatively common, we also see it in はいんな → 入んな on page 70 and 71.
Quoting myself, if I may:
We see here that やっていられない becomes やってられない and finally やてらんない.
So, we can gather that R sounds, that is, れ and る (at least), become ん before N sounds.
It’s a way to say less, I’d say, because of how sounds work.
Examples
In this case, he’s saying, “I will go to the convenience store, okay?” Instead of 来るな as you would think.
We see it twice here, in 何すんだ being 何するんだ, and 入んな again. 
Here, for example, he’s saying, “Why are you eating cup ramen in my house?”
The important part here is 食ってんだ → 食っているんだ
もうかってんの？ → もう買っているの？ “Did you buy it already?”
This applies to most other cases as well:
We have 買っているの drop the い → 買ってるの, then る becomes ん to form 買ってんの.
There are a lot of examples.
うめ is another like すげー, うまい → うめー, すごい → すげー, etc.