fixed vdh bug when trying to extract images from invalid video frames

[keighrim]

fixes #400

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.21%. Comparing base (ecc064d) to head (1326027).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
mmif/utils/video_document_helper.py	94.44%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #403      +/-   ##
===========================================
+ Coverage    75.04%   78.21%   +3.16%     
===========================================
  Files           27       27              
  Lines         3274     3369      +95     
===========================================
+ Hits          2457     2635     +178     
+ Misses         817      734      -83     

Flag	Coverage Δ
unittests	78.21% <94.44%> (+3.16%)	⬆️

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