Add fuzzer issues

[niccokunzmann]

Icalendar was added to oss-fuzz in #562. #587 adds a way to reproduce the issues the fuzzer found.

To close this issue:

• list all fuzzer issues as separate issues or as a listing
• (Ref1) close this issue by creating a PR that shows how to reproduce and fix a fuzzer issue.
• document who has access to these issues in https://icalendar.readthedocs.io/en/latest/maintenance.html
• document fuzzer issues link somewhere
  • also document how to solve them
    • include links to this issue and the PR (Ref1)

References:

• OSS-Fuzz Integration #562
• Add scripts and tests to reproduce fuzzer errors in pytest #587
• Public, neglected issues: https://issues.oss-fuzz.com/issues?q=is:open%20icalendar (login allows more)