Update container.py to solve issue 224

[tongwenchao]

This change solves an error "unpack requires a buffer of 4 bytes". https://github.com/google/spatial-media/issues/224

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[tongwenchao]

@googlebot I signed it!

[Fabrice-TIERCELIN]

I confirm this solve the problem.

[jamiesonpepper]

Merged as requested, thanks for the contribution!