Fix stable_math::compute_invariant panics on a zero balance

[johngrantuk]

5. stable_math::compute_invariant panics on a zero balance

for balance in balances {
    d_p = (d_p * invariant) / (balance * U256::from(num_tokens));
}

Any zero balance divides by zero. Reachable for a pool with three or more tokens where one has been
drained: a caller screening only the two tokens being swapped still hands the whole balance array to
this function.

Suggested fix: return PoolError when any balance is zero — the invariant is undefined there.