Stableswap share spot price calculation

[martinfridrich]

This part of the code is not correct. Stableswap share is not part of the assets in stableswap so this check prevent spot price calculation for shares

HydraDX-wasm/src/lib.rs

Lines 749 to 753 in d0e8a31

	let idx_in = reserves.iter().position(|v| v.asset_id == asset_in);
	let idx_out = reserves.iter().position(|v| v.asset_id == asset_out);
	if idx_in.is_none() || idx_out.is_none() {
	return error();
	}