Core: stricter UTF-8 handling in ngx_utf8_decode(). An UTF-8 octet sequence cannot start with a 11111xxx byte (above 0xf8), see https://datatracker.ietf.org/doc/html/rfc3629#section-3. Previously, such bytes were accepted by ngx_utf8_decode() and misinterpreted as 11110xxx bytes (as in a 4-byte sequence). While unlikely, this can potentially cause issues. Fix is to explicitly reject such bytes in ngx_utf8_decode().