Fix NPE with watching NatvisSection (#1378)

This PR fixes the variable used for the RegistryMonitor on a fresh VS
instance.

If the private registry does not have a NatvisDiagnostic section, it
will watch for its creation then delete this watcher.
However when creating the new watcher it would watch on a null section
instead of the new natvis section.

This PR fixes that issue.

Author: WardenGnaw

src/DebugEngineHost/HostNatvisProject.cs

@@ -104,7 +104,7 @@ public static IDisposable WatchNatvisOptionSetting(HostConfigurationStore config
                             IDisposable disposable = rmw.CurrentMonitor;
 
                             // Watch NatvisDiagnostic section
-                            rmw = new RegisterMonitorWrapper(CreateAndStartNatvisDiagnosticMonitor(natvisDiagnosticSection, natvisLogger));
+                            rmw = new RegisterMonitorWrapper(CreateAndStartNatvisDiagnosticMonitor(checkForSection, natvisLogger));
 
                             disposable.Dispose();
                         }