SimplePie Fix regex backtrack limit in clean_hash() (#7813)
https://github.com/FreshRSS/simplepie/pull/48
and merge upstream https://github.com/FreshRSS/simplepie/pull/49
fix https://github.com/FreshRSS/FreshRSS/issues/7807
We had a risk of hitting `Backtrack limit was exhausted` in case of open XML comment `<!--` not closed and followed by a very long document.
Fixed by:
* Limiting the max length of the comment
* Stopping at an apparent end of CDATA section `]]>` as it is likely an error
It does not matter much if there are rare cases when the regex does not work perfectly, as it is only used for a cache hint.