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.