[x/gamm][stableswap][bug]: Enforce minimum post-scaled asset amounts

[AlpinYukseloglu]

Background

Since our k values are cubic in the input amounts, scaling factors that take input reserves below 1 expose an attack vector where our solver can be forced to panic (push reserves to be small enough such that k is always zero causing a div by zero when calculating k ratios)

Suggested Design

• Require a minimum post-scaled asset amount of 1 per asset in a pool and enforce this on pool creation and pool exit (both should fail if this requirement is not met)

Acceptance Criteria

• All new and existing tests pass