Fix value error and key error

[tongbaojia]

• Fixes issue ValueError and KeyErrors in PDFDocument #573
• Two things: check the stack length before popping, and also use .get to protect getting a key that doesn't exist.
  The code change is pretty simple and in general good practice in python so I think they should be adopted.
  Sorry that I couldn't share the original PDF document as explained in the issue.
• Issue number ValueError and KeyErrors in PDFDocument #573

How Has This Been Tested?
Can confirm that with these fixes the document can be processed.

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[pietermarsman]

I think this looks good and harmless. Can you add a FIxes line to the CHANGELOG.md?

[tongbaojia]

I updated the CHANGELOG. but @pietermarsman something is wrong with the bot running the test?

[tongbaojia]

The bot raises:

py36 inst: /home/travis/build/pdfminer/pdfminer.six/.tox/.tmp/package/1/pdfminer.six-20201018.zip
ERROR: invocation failed (exit code 1), logfile: /home/travis/build/pdfminer/pdfminer.six/.tox/py36/log/py36-1.log

[pietermarsman]

Can you merge/rebase with the develop branch? This will fix the cicd failure.

[tongbaojia]

@pietermarsman is this PR ready to merge? Thanks :)