Add a test case for usage of closed connection

After 2d55559, we are setting some fields to nil in the shutdown
sequence. We have to ensure that the library does not panic as a
consequence, after the connection is closed.

Signed-off-by: Aitor Perez Cedres <acedres@vmware.com>

client_test.go

@@ -288,7 +288,41 @@ func TestOpen(t *testing.T) {
 	if c, err := Open(rwc, defaultConfig()); err != nil {
 		t.Fatalf("could not create connection: %v (%s)", c, err)
 	}
+}
+
+func TestOpenClose_ShouldNotPanic(t *testing.T) {
+	rwc, srv := newSession(t)
+	t.Cleanup(func() {
+		_ = rwc.Close()
+	})
+
+	go func() {
+		srv.connectionOpen()
+		srv.connectionClose()
+	}()
+
+	c, err := Open(rwc, defaultConfig())
+	if err != nil {
+		t.Fatalf("could not create connection: %v (%s)", c, err)
+	}
+
+	if err := c.Close(); err != nil {
+		t.Fatalf("could not close connection: %s", err)
+	}
 
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("creating a channel on a closed connection should not panic: %s", r)
+		}
+	}()
+
+	ch, err := c.Channel()
+	if ch != nil {
+		t.Fatalf("creating a channel on a closed connection should not succeed: %v, (%s)", ch, err)
+	}
+	if err != ErrClosed {
+		t.Fatalf("error should be closed: %s", err)
+	}
 }
 
 func TestChannelOpen(t *testing.T) {