Assert that assetData is a string before we try to decode it

packages/asset-buyer/src/asset_buyer.ts

@@ -193,6 +193,7 @@ export class AssetBuyer {
     ): Promise<LiquidityForAssetData> {
         const shouldForceOrderRefresh =
             options.shouldForceOrderRefresh !== undefined ? options.shouldForceOrderRefresh : false;
+        assert.isString('assetData', assetData);
         assetDataUtils.decodeAssetDataOrThrow(assetData);
         assert.isBoolean('options.shouldForceOrderRefresh', shouldForceOrderRefresh);