allow shutdown of unstarted server

src/csharp/Grpc.Core.Tests/ServerTest.cs

@@ -94,6 +94,14 @@ namespace Grpc.Core.Tests
             server.ShutdownAsync().Wait();
         }
 
+        [Test]
+        public void UnstartedServerCanBeShutdown()
+        {
+            var server = new Server();
+            server.ShutdownAsync().Wait();
+            Assert.Throws(typeof(InvalidOperationException), () => server.Start());
+        }
+
         [Test]
         public void UnstartedServerDoesNotPreventShutdown()
         {

src/csharp/Grpc.Core/Server.cs

@@ -140,6 +140,7 @@ namespace Grpc.Core
             lock (myLock)
             {
                 GrpcPreconditions.CheckState(!startRequested);
+                GrpcPreconditions.CheckState(!shutdownRequested);
                 startRequested = true;
                 
                 handle.Start();
@@ -203,7 +204,6 @@ namespace Grpc.Core
         {
             lock (myLock)
             {
-                GrpcPreconditions.CheckState(startRequested);
                 GrpcPreconditions.CheckState(!shutdownRequested);
                 shutdownRequested = true;
             }
@@ -215,7 +215,6 @@ namespace Grpc.Core
             {
                 handle.CancelAllCalls();
             }
-
             await ShutdownCompleteOrEnvironmentDeadAsync().ConfigureAwait(false);
 
             DisposeHandle();