Fix invalid assertion in TcpListener

Author: drewnoakes

src/NetMQ/Core/Transports/Tcp/TcpListener.cs

@@ -123,7 +123,7 @@ public virtual void SetAddress(string addr)
             m_address.Resolve(addr, m_options.IPv4Only);
 
             Assumes.NotNull(m_address.Address);
-            Assumes.NotNull(m_handle);
+            Assumes.Null(m_handle);
 
             try
             {

src/NetMQ/Utils/Assumes.cs

@@ -9,8 +9,14 @@ internal static class Assumes
         [Conditional("DEBUG")]
         public static void NotNull<T>([NotNull] T o) where T : class?
         {
-            Debug.Assert(o is object, $"Unexpected null of type {typeof(T).Name}");
+            Debug.Assert(o is not null, $"Unexpected null value of type {typeof(T).Name}");
         }
 #pragma warning restore CS8777 // Parameter must have a non-null value when exiting.
+
+        [Conditional("DEBUG")]
+        public static void Null<T>([MaybeNull] T o) where T : class?
+        {
+            Debug.Assert(o is null, $"Unexpected non-null value of type {typeof(T).Name}");
+        }
     }
 }