Handle null reference exception during analyzer execution when cancellation has been requested

Fixes VSO Watson [#763828](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/763828)
All the dumps for the watson show that cancellation was requested during analysis, leading to one thread clearing state while other thread faulted.

src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerDriver.cs

@@ -1853,6 +1853,11 @@ private static void ComputeDeclarationsInNode(SemanticModel semanticModel, ISymb
             // Mark completion if we successfully executed all actions and only if we are analyzing a span containing the entire syntax node.
             if (success && analysisStateOpt != null && !declarationAnalysisData.IsPartialAnalysis)
             {
+                // Ensure that we do not mark declaration complete/clear state if cancellation was requested.
+                // Other thread(s) might still be executing analysis, and clearing state could lead to corrupt execution
+                // or unknown exceptions.
+                cancellationToken.ThrowIfCancellationRequested();
+
                 foreach (var analyzer in analysisScope.Analyzers)
                 {
                     analysisStateOpt.MarkDeclarationComplete(symbol, declarationIndex, analyzer);