Export exeption with empty session

Situation no opened session:

export -z test.zip
[!] The command export raised an exception:
Traceback (most recent call last):
  File "/home/jaegeral/viper/viper/core/ui/console.py", line 195, in start
    self.cmd.commands[root]['obj'](*args)
  File "/home/jaegeral/viper/viper/core/ui/commands.py", line 878, in cmd_export
    export_zip.write(__sessions__.current.file.path, arcname=__sessions__.current.file.name)

--> check if session is opened
Merge pull request #108 from deralexxx/patch-7

Export exeption with empty session