NRE in TryFindMissingDependencies

[jakebailey]

Microsoft.Python.Analysis.Dependencies.DependencyResolver`2.TryFindMissingDependencies(ImmutableArray`1&, ImmutableArray`1&, Int32) in Analysis\Ast\Impl\Dependencies\DependencyResolver.cs:line 563
Microsoft.Python.Analysis.Dependencies.DependencyResolver`2.TryCreateWalker(Int32, Int32, IDependencyChainWalker`2&) in Analysis\Ast\Impl\Dependencies\DependencyResolver.cs:line 214
Microsoft.Python.Analysis.Analyzer.PythonAnalyzer.TryCreateSession(Int32, PythonAnalyzerEntry, PythonAnalyzerSession&) in Analysis\Ast\Impl\Analyzer\PythonAnalyzer.cs:line 246
Microsoft.Python.Analysis.Analyzer.PythonAnalyzer.AnalyzeDocument(AnalysisModuleKey, PythonAnalyzerEntry, ImmutableArray`1) in Analysis\Ast\Impl\Analyzer\PythonAnalyzer.cs:line 224

Line 563 is just version != _version, neither of which can ever be null (they're int!!!). Happening just as often as the GetBodyStartIndex NRE.

cc Alexander Sher (@AlexanderSher)