Update arc.js

Fix for issue #1205.

I added a console.log while debugging this to verify that this null condition was being hit and that the charts still generated correctly after hitting this.

src/arc.js

@@ -26,8 +26,9 @@ c3_chart_internal_fn.updateArc = function () {
 
 c3_chart_internal_fn.updateAngle = function (d) {
     var $$ = this, config = $$.config,
-        found = false, index = 0,
-        gMin = config.gauge_min, gMax = config.gauge_max, gTic, gValue;
+        found = false, index = 0;
+    if (config) {  
+        var gMin = config.gauge_min, gMax = config.gauge_max, gTic, gValue;
         $$.pie($$.filterTargetsToShow($$.data.targets)).forEach(function (t) {
             if (! found && t.data.id === d.data.id) {
                 found = true;
@@ -48,6 +49,7 @@ c3_chart_internal_fn.updateAngle = function (d) {
             d.startAngle = -1 * (Math.PI / 2);
             d.endAngle = d.startAngle + gTic * gValue;
         }
+    }
     return found ? d : null;
 };