BufferGeometry: Added box.makeEmpty() in computeBoundingSphere. Fixes #4277.

src/core/BufferGeometry.js

@@ -167,6 +167,8 @@ THREE.BufferGeometry.prototype = {
 
 			if ( positions ) {
 
+				box.makeEmpty();
+
 				var center = this.boundingSphere.center;
 
 				for ( var i = 0, il = positions.length; i < il; i += 3 ) {