Add defensive assertion to vdev_queue_aggregate()

a6ccb36 had been intended to include
this to silence Coverity reports, but this one was missed by mistake.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14043

module/zfs/vdev_queue.c

@@ -725,6 +725,7 @@ vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
 		 * after our span is mandatory.
 		 */
 		dio = AVL_NEXT(t, last);
+		ASSERT3P(dio, !=, NULL);
 		dio->io_flags &= ~ZIO_FLAG_OPTIONAL;
 	} else {
 		/* do not include the optional i/o */