WebGLGeometries: Delete index on geometry dispose. See #7389.

src/renderers/webgl/WebGLGeometries.js

@@ -49,6 +49,12 @@ THREE.WebGLGeometries = function ( gl, properties, info ) {
 		var geometry = event.target;
 		var buffergeometry = geometries[ geometry.id ];
 
+		if ( buffergeometry.index !== null ) {
+
+			deleteAttribute( buffergeometry.index );
+
+		}
+
 		deleteAttributes( buffergeometry.attributes );
 
 		geometry.removeEventListener( 'dispose', onGeometryDispose );