Avoid crashes in skeleton

scene/3d/skeleton.cpp

@@ -160,14 +160,14 @@ void Skeleton::_notification(int p_what) {
 
 			//if moved, just update transforms
 			VisualServer *vs = VisualServer::get_singleton();
-			Bone *bonesptr = &bones[0];
+			const Bone *bonesptr = bones.ptr();
 			int len = bones.size();
 			Transform global_transform = get_global_transform();
 			Transform global_transform_inverse = global_transform.affine_inverse();
 
 			for (int i = 0; i < len; i++) {
 
-				Bone &b = bonesptr[i];
+				const Bone &b = bonesptr[i];
 				vs->skeleton_bone_set_transform(skeleton, i, global_transform * (b.transform_final * global_transform_inverse));
 			}
 		} break;