Improve Box3 closure performance

src/math/Box3.js

@@ -300,12 +300,10 @@ Object.assign( Box3.prototype, {
 
 	intersectsSphere: ( function () {
 
-		var closestPoint;
+		var closestPoint = new Vector3();
 
 		return function intersectsSphere( sphere ) {
 
-			if ( closestPoint === undefined ) closestPoint = new Vector3();
-
 			// Find the point on the AABB closest to the sphere center.
 			this.clampPoint( sphere.center, closestPoint );