Calling the toJSONString method with UIColor.clear will result in an error

### Question

Calling toJSONString after creating an object with the UIColor.clear property reports an error and fails to serialize successfully.

### Code

```swift
class SandBoxModel: HandyJSON {
var mode: Int = 0
var color: UIColor = .clear

func mapping(mapper: HelpingMapper) {
mapper <<<
color <-- HexColorTransform()
}

public required init() {}
}

var model = SandBoxModel()
model.color = .clear
if let jsonString = model.toJSONString {
print(jsonString)
}
```

### Error

`Swift/ContiguousArrayBuffer.swift:580: Fatal error: Index out of range`

Reproduce issue #486 with the provided SandBoxModel, UIColor.clear, and HexColorTransform, starting at the toJSONString entry point. Trace the serialization path for the clear color and compare it with other UIColor values. Done means the example no longer triggers an index-out-of-range failure and serializes successfully.

Written by the indexing model from the issue text.