If select() is passed another cfn function, any index greater than 0 fails


Specific case: selecting one of the availability zones returned from get_azs():

```
r.add(Resource('Subnet1',
    'AWS::EC2::Subnet',
    {
        'AvailabilityZone': select('1', get_azs()),
        'VpcId': ref('VPC'),
        'CidrBlock': 'CIDR1',
    }))
```

throws:
```
cfn_pyplates.exceptions.IntrinsicFuncInputError: Provided index is invalid!
```