sshd: shared reference in JGitClientSession must be volatile

The proxy handler may be re-set from an unspecified I/O thread.
Declare the shared variable as volatile.

Change-Id: I4e7ce393ae2cdc7f1cd4edf40c137da6d6c50ad5
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit.ssh.apache/src/org/eclipse/jgit/internal/transport/sshd/JGitClientSession.java

@@ -98,7 +98,7 @@ public class JGitClientSession extends ClientSessionImpl {
 
 	private CredentialsProvider credentialsProvider;
 
-	private StatefulProxyConnector proxyHandler;
+	private volatile StatefulProxyConnector proxyHandler;
 
 	/**
 	 * @param manager