chore: prevent creating stable pool when a=0

[gregdhill]

Signed-off-by: Gregory Hill [email protected]

Errors occur here:

interbtc/crates/dex-stable/src/utils.rs

Lines 41 to 53 in 1f75334

	let denominator = ann
	.checked_sub(a_precision)
	.and_then(|n| n.checked_mul(d))
	.and_then(|n| n.checked_div(a_precision))
	.and_then(|n| {
	n.checked_add(
	U256::from(n_currencies)
	.checked_add(U256::from(1u32))?
	.checked_mul(d_p)?,
	)
	})?;
	d = numerator.checked_div(denominator)?;

interbtc/crates/dex-stable/src/utils.rs

Lines 90 to 95 in 1f75334

	c = c
	.checked_mul(d)?
	.checked_mul(a_percision)?
	.checked_div(ann.checked_mul(n_currencies)?)?;
	let b = sum.checked_add(d.checked_mul(a_percision)?.checked_div(ann)?)?;

Same issue in Acala implementation: https://github.com/nutsfinance/stable-asset/blob/9a60645bc60e1c3452e8313de43179c4ba9b6e36/lib/stable-asset/src/lib.rs#L844

Related: https://github.com/saber-hq/stable-swap/blob/c621f5950e8071aaa595bc9ad674e970fccebdb2/stable-swap-math/src/curve.rs#L22