Nested clozes generate some weird html, cause they render the inner cloze and then escape it to put in the data-cloze attribute. This results in weird html in attribute:

<span class="cloze" data-cloze="<span class=&quot;cloze&quot; data-cloze=&quot;&quot; data-ordinal=&quot;1&quot;>[...]</span>" data-ordinal="1">[...]</span>

and that html contains nested html escapes that grow exponentially. This huge html chunk slowes down extract_av_tags to a crawl and crashes anki.

This starts to cause crashes with ~11 nested clozes. Some shared decks on ankiweb have those and a user might make them by mistake, so there should be some kind of safeguard.

Originally reported on https://forums.ankiweb.net/t/memory-leak-when-search-specific-kanji-japanese-hieroglyph/44394/2