prune_io_piece_log() also needs to take list into account

If we didn't use the rbtree but have remaining entries to prune,
loop over the list as well and free those entries.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/log.c b/log.c
index e7320ef..8754eb0 100644
--- a/log.c
+++ b/log.c
@@ -108,6 +108,12 @@
 		rb_erase(n, &td->io_hist_tree);
 		free(ipo);
 	}
+
+	while (!list_empty(&td->io_hist_list)) {
+		ipo = list_entry(td->io_hist_list.next, struct io_piece, list);
+		list_del(&ipo->list);
+		free(ipo);
+	}
 }
 
 /*