fixed isWithinBar() crash when there were less than 2 items in pathSegList

src/shape.bar.js

@@ -113,6 +113,9 @@ c3_chart_internal_fn.generateGetBarPoints = function (barIndices, isSub) {
     };
 };
 c3_chart_internal_fn.isWithinBar = function (that) {
+    if (that.pathSegList.numberOfItems < 2) {
+        return false;
+    }
     var mouse = this.d3.mouse(that), box = that.getBoundingClientRect(),
         seg0 = that.pathSegList.getItem(0), seg1 = that.pathSegList.getItem(1),
         x = Math.min(seg0.x, seg1.x), y = Math.min(seg0.y, seg1.y),