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The very short answer is that this is not a double negative.

The reason is that the negatives don't appear in the same clause. If I said, "I don't like creed, and I'm not a particularly big fan of Nickleback," for example, it does not mean that I like creed and am a particularly big fan of Nickleback. Similarly, if I made a different conditional statement like "If I can't get a ride, then I won't be able to go out," then I'm saying that on the condition of one thing not happening, something else won't happen. It MAY be the case that if it DOES happen, then the something else WILL necessarily happen; whether it's logically the case or not (as I'll cover in a second, it's not anyway) that's just not what you're saying when you're using a conditional with a negative in each clause.

As for the rest of your post, it kinda gets into the logic of it more than the grammar of it. Saying that "if negative, then negative" necessarily means "if positive, then positive" is a syllogistic fallacy--a pretty common one, too. As an example, this logic would mean that I could conclude that whales and dolphins and all sorts of bacteria are fish:

Sub-Premise: If a species does not spend most of its life in water, then it is not a fish.
Fallacious Premise: if a species does spend most of its life then it's a fish.
Premise: whales spend most of their lives in water.
Conclusion: whales are fish.

As for the slang of it, this is how I, and I think just about everyone else, reads it. Being **** is bad, but if "it ain't ****," then it's worthless. So if it is not pit, then it's worthless. So even though this doesn't necessarily mean that everything that IS pit is worth-ful...but according to this slogan, it's the only chance you got.