cea608overlay: ensure tha the list of lines remains sorted by line number

Otherwise, when iterating over the lines, they may be written out of order.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/2051>

video/closedcaption/src/cea608utils.rs

@@ -191,7 +191,8 @@ impl Cea608Frame {
                 line: VecDeque::new(),
                 initial_preamble: None,
             });
-            lines.back_mut().unwrap()
+            lines.make_contiguous().sort_by_key(|line| line.no);
+            lines.iter_mut().find(|line| line.no == row).unwrap()
         };
         while line.line.len() <= column {
             line.line.push_back(Cea608Cell::Empty);