meatureText may return `undefined` length for first line while the width of the first word out of the given width

https://github.com/Flipboard/react-canvas/blob/master/lib/measureText.js#L75

``` js
if (textMetrics.width > width || (lastBreak && lastBreak.required)) {
measuredSize.height += lineHeight;
measuredSize.lines.push({width: lastMeasuredWidth, text: currentLine.trim()});
```

Should be:

``` js
if (textMetrics.width > width || (lastBreak && lastBreak.required)) {
if (currentLine.length > 0) {
measuredSize.height += lineHeight;
measuredSize.lines.push({width: lastMeasuredWidth, text: currentLine.trim()});
}
```