innerRadius has Nan value for gauge type

* **C3 version**: 0.7.1
* **D3 version**: 5.8.0
* **Browser**: Google Chrome
* **OS**: macOS 10.14.5

Error: attribute dx: Expected length, "NaNpx".
Replication:
- Happens every time lost focus from/to browser tab.

Investigation:
- w = config.gauge_width is has assigned value 10
- $$.innerRadiusRatio is -Infinity
- $$.innerRadius is NaN

Quick Fix in line 5804:
$$.innerRadius = $$.innerRadius || 0;