Its possible to call the method addAsset multiple times with the same _underlying, if you call addAsset with empty symbol it will bypass the validAsset modifier, and it will be possible to call again the addAsset with different values. It will produce a mismatch with the reality and getAssetsLength.
This could cause a loss of funds if it is not verified before that this symbol is other than empty. It is mandatory to add a require to verify that the symbol is not empty. Its also recommended to add a require in _assetSymbol to ensure non-existence tokens are returned.
