I have always wondered about that, and we've discussed it at length in other forums. Couldn't understand if it was an old hold out from the "ne pas", or a shortening (and maybe misuse?) of "y en a pas..". Regardless, I heard it a lot growing up.
Yes, the liaison (y en a / y en a pas) or the negative with n' (y en n'a pas) make the same sound. Both probably merged into the "i-n-a / i-n-a pas" form. (I'm just guessing).