Avoid throwing exception when attempting to shutdown a non-open connection

Author: Scott Tadman

lib/net/sftp.rb

@@ -40,7 +40,7 @@ def self.start(host, user, options={}, &block)
       sftp
     rescue Object => anything
       begin
-        session.shutdown!
+        session and session.shutdown!
       rescue Exception
         # swallow exceptions that occur while trying to shutdown
       end