Null ref in PythonAnalyzerSession.Analyze

[MikhailArkhipov]

No reliable repro, happened in tests. Looks like _walker became null.

 System.NullReferenceException: Object reference not set to an instance of an object.
       at void Microsoft.Python.Analysis.Analyzer.PythonAnalyzerSession.Analyze(IDependencyChainNode<PythonAnalyzerEntry> node, AsyncCountdownEvent ace, Stopwatch stopWatch) in F:/PLS/src/Analysis/Ast/Impl/Analyzer/PythonAnalyzerSession.cs:line 290
       at Task Microsoft.Python.Analysis.Analyzer.PythonAnalyzerSession.StartAnalysis(IDependencyChainNode<PythonAnalyzerEntry> node, AsyncCountdownEvent ace, Stopwatch stopWatch)+() => { } in F:/PLS/src/Analysis/Ast/Impl/Analyzer/PythonAnalyzerSession.cs:line 243
       at void System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref Task currentTaskSlot)

[MikhailArkhipov]

#1482 fixes this.