fix: UnhandledException: System.ObjectDisposedException. (#1590)

* fix: UnhandledException: System.ObjectDisposedException.
Error on thread pool

#1588

* tweak message

---------

Co-authored-by: Rob Hague <rob.hague00@gmail.com>

src/Renci.SshNet/BaseClient.cs

@@ -502,6 +502,14 @@ namespace Renci.SshNet
                 {
                     _ = session.TrySendMessage(new IgnoreMessage());
                 }
+                catch (ObjectDisposedException)
+                {
+                    // ignore
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogError(ex, "Error sending keepalive message");
+                }
                 finally
                 {
                     Monitor.Exit(_keepAliveLock);