[imp] IMP makes Horde unusable if use_notepad set to true

[Aleksandar Milivojevic]

If use_notepad is set to true in IMP 4.1, IMP forces horde to  
redisplay login page in main frame, making entire system unusable  
(nothing can be accessed, including calendar, addressbook, filters,  
admin, ...).  The HTTP log file shows:

PHP Notice:  Undefined index:  notepad in  
/usr/share/horde/imp/lib/Session.php on line 232, referer:  
https://localhost/horde/index.php

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