avoid errors if the duration is undefined or null

Author: crisbeto

src/roundProgress.js

@@ -95,7 +95,7 @@ angular.module('angular-svg-round-progress')
 
                         var easingAnimation     = service.animations[options.animation];
                         var startTime           = new Date();
-                        var duration            = parseInt(options.duration);
+                        var duration            = parseInt(options.duration) || 0;
                         var preventAnimation    = (newValue > max && oldValue > max) || (newValue < 0 && oldValue < 0) || duration < 25;
 
                         var radius              = options.radius;