e window list: prevent from segfaulting, patch from andrej.

 e window list: prevent from segfaulting, patch from andrej.

 Description from Trac (see #1020):

Pressing a key which is bound to "next/previous window of the same class" can make Enlightenment SEGV. This happens when:

    Window switcher list module is loaded
    the key is pressed when the active desktop is empty (i.e., contains no windows / applications) and at least one other desktop contains one or several windows

This does not happen if all the desktops are empty.

 Fixes #1020.

SVN revision: 72753