Add null pointer checks to avoid segfaults

[nhz2]

Fixes #70

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.76%. Comparing base (e7edfed) to head (f399913).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   61.55%   62.76%   +1.20%     
==========================================
  Files           5        5              
  Lines         372      384      +12     
==========================================
+ Hits          229      241      +12     
  Misses        143      143              

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[mkitti]

Add tests

[mkitti]

Use @test_throws?
https://docs.julialang.org/en/v1/stdlib/Test/#Test.@test_throws

[nhz2]

I am avoiding testing specific behavior of these functions, since after finalize, their behavior is undocumented and may change in the future. Specifically, instead of erroring, initialize could just re allocate the codec, or be a noop.

[nhz2]

Closing in favor of #74
If we are already checking for null, why error if we can just initialize again.