Don't return SslStreamBuffer when disposing during handshake

[rzikm]

Fixes #66818.

This PR makes sure we don't dispose buffers which may be still in use by ongoing handshake, which on debug builds led to an assert firing and crashing entire test sutite (see #65455 for example).

Dispose during ongoing read operations was handled correctly before.

@wfurt you mentioned we should have some tests which dispose the SslStream mid-operation on purpose. Would something like below suffice? It is essentially what we did before #66682.

        SslProtocols clientProtocol;
        SslProtocols serverProtocol;

        clientProtocol = SslProtocols.Tls13;
        serverProtocol = SslProtocols.Tls12;

        TcpListener listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();

        string certPath = Path.Join(new FileInfo(Assembly.GetExecutingAssembly().Location).Directory.FullName, "contoso.com.pfx");
        System.Console.WriteLine(certPath);
        using X509Certificate2 serverCertificate = new X509Certificate2(File.ReadAllBytes(certPath), "testcertificate");

        while (true)
        {
            System.Console.Write('.');
            using (TcpClient client = new TcpClient())
            {
                Task clientConnectTask = client.ConnectAsync(IPAddress.Loopback, ((IPEndPoint)listener.LocalEndpoint).Port);
                Task<TcpClient> listenerAcceptTask = listener.AcceptTcpClientAsync();

                await Task.WhenAll(clientConnectTask, listenerAcceptTask);

                TcpClient server = listenerAcceptTask.Result;
                using (SslStream clientStream = new SslStream(
                    client.GetStream(),
                    false,
                    new RemoteCertificateValidationCallback(delegate { return true; }),
                    null,
                    EncryptionPolicy.RequireEncryption))
                using (SslStream serverStream = new SslStream(
                    server.GetStream(),
                    false,
                    null,
                    null,
                    EncryptionPolicy.RequireEncryption))
                {

                    Task clientAuthenticationTask = clientStream.AuthenticateAsClientAsync(
                        serverCertificate.GetNameInfo(X509NameType.SimpleName, false),
                        null,
                        clientProtocol,
                        false);

                    try
                    {
                        await serverStream.AuthenticateAsServerAsync(
                           serverCertificate,
                           false,
                           serverProtocol,
                           false);
                    }
                    catch (AuthenticationException)
                    {
                    }

                    // e = await Assert.ThrowsAsync<AuthenticationException>(() => clientAuthenticationTask);
                }
            }
        }

</details/

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Issue Details

---

Fixes #66818.

This PR makes sure we don't dispose buffers which may be still in use by ongoing handshake, which on debug builds led to an assert firing and crashing entire test sutite (see #65455 for example).

Dispose during ongoing read operations was handled correctly before.

@wfurt you mentioned we should have some tests which dispose the SslStream mid-operation on purpose. Would something like below suffice? It is essentially what we did before #66682.

```cs SslProtocols clientProtocol; SslProtocols serverProtocol;

    clientProtocol = SslProtocols.Tls13;
    serverProtocol = SslProtocols.Tls12;

    TcpListener listener = new TcpListener(IPAddress.Loopback, 0);
    listener.Start();

    string certPath = Path.Join(new FileInfo(Assembly.GetExecutingAssembly().Location).Directory.FullName, "contoso.com.pfx");
    System.Console.WriteLine(certPath);
    using X509Certificate2 serverCertificate = new X509Certificate2(File.ReadAllBytes(certPath), "testcertificate");

    while (true)
    {
        System.Console.Write('.');
        using (TcpClient client = new TcpClient())
        {
            Task clientConnectTask = client.ConnectAsync(IPAddress.Loopback, ((IPEndPoint)listener.LocalEndpoint).Port);
            Task<TcpClient> listenerAcceptTask = listener.AcceptTcpClientAsync();

            await Task.WhenAll(clientConnectTask, listenerAcceptTask);

            TcpClient server = listenerAcceptTask.Result;
            using (SslStream clientStream = new SslStream(
                client.GetStream(),
                false,
                new RemoteCertificateValidationCallback(delegate { return true; }),
                null,
                EncryptionPolicy.RequireEncryption))
            using (SslStream serverStream = new SslStream(
                server.GetStream(),
                false,
                null,
                null,
                EncryptionPolicy.RequireEncryption))
            {

                Task clientAuthenticationTask = clientStream.AuthenticateAsClientAsync(
                    serverCertificate.GetNameInfo(X509NameType.SimpleName, false),
                    null,
                    clientProtocol,
                    false);

                try
                {
                    await serverStream.AuthenticateAsServerAsync(
                       serverCertificate,
                       false,
                       serverProtocol,
                       false);
                }
                catch (AuthenticationException)
                {
                }

                // e = await Assert.ThrowsAsync<AuthenticationException>(() => clientAuthenticationTask);
            }
        }
    }

</details/

<table>
  <tr>
    <th align="left">Author:</th>
    <td>rzikm</td>
  </tr>
  <tr>
    <th align="left">Assignees:</th>
    <td>-</td>
  </tr>
  <tr>
    <th align="left">Labels:</th>
    <td>

`area-System.Net.Security`

</td>
  </tr>
  <tr>
    <th align="left">Milestone:</th>
    <td>-</td>
  </tr>
</table>
</details>

[wfurt]

LGTM.
We will not return buffer to the shared pool in that case but it seems ok since that should be rare. (it was not issue prior #64747 as we did not rent buffer for handshake)
cc @stephentoub for any additional concerns.

[rzikm]

Test failures are unrelated