Fix NullReferenceException in EE

I made a struct into a class (in 0d415c29) but I missed a null check
around an existing member access.

src/ExpressionEvaluator/Core/Source/ExpressionCompiler/DkmUtilities.cs

@@ -132,13 +132,12 @@ internal static object GetSymReader(this DkmClrModuleInstance clrModule)
             ResultProperties resultProperties,
             DkmClrRuntimeInstance runtimeInstance)
         {
-            if (compResult.Assembly == null)
+            if (compResult == null)
             {
-                Debug.Assert(compResult.TypeName == null);
-                Debug.Assert(compResult.MethodName == null);
                 return null;
             }
 
+            Debug.Assert(compResult.Assembly != null);
             Debug.Assert(compResult.TypeName != null);
             Debug.Assert(compResult.MethodName != null);