ssh: prevent a logic error that could result in an infinite loop

lib/ssh.c

@@ -2204,7 +2204,8 @@ static CURLcode ssh_statemach_act(struct connectdata *conn, bool *block)
       /* Check if nextstate is set and move .nextstate could be POSTQUOTE_INIT
          After nextstate is executed,the control should come back to
          SSH_SFTP_CLOSE to pass the correct result back  */
-      if(sshc->nextstate != SSH_NO_STATE) {
+      if(sshc->nextstate != SSH_NO_STATE &&
+         sshc->nextstate != SSH_SFTP_CLOSE) {
         state(conn, sshc->nextstate);
         sshc->nextstate = SSH_SFTP_CLOSE;
       }