subparse: Look for the closing `>` of a tag after the opening `<`

subparse: Look for the closing `>` of a tag after the opening `<`

Previously when fixing up subrip markip, we were looking from the start
of the remaining buffer instead. Due to how skipping over closing tags
works, the remaining buffer will still contain the closing `>` of the
previous tag so if a unexpected closing tag is found after another
closing tag, we would potentially do an out of bounds memmove().

Fixes ZDI-CAN-20968
Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2662

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4898>