Safely cast object id to integer in PDFParser to prevert TypeError and ValueError.

[pietermarsman]

Pull request

Fixes #971.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=70071

Not assume that object can be converted to int. But use try-except in safe_int and return None if it can't be converted to int. Simply ignore the object if that is the case.

How Has This Been Tested?

With the nox tests. And with the fuzz blob.

Checklist

• [x] I have read CONTRIBUTING.md.
• [x] I have added a concise human-readable description of the change to CHANGELOG.md.
• [x] I have tested that this fix is effective or that this feature works.
• [x] I have added docstrings to newly created methods and classes.
• [x] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.