Fix crash in mainwindows_resize_smaller when the 'mainwindows' list is empty.

The crash can happen if the terminal height decreases before the first window
is created. Based on a patch by Jaroslav Škarvada (red hat bug #796457).


git-svn-id: http://svn.irssi.org/repos/irssi/trunk@5217 dbcabf3a-b0e7-0310-adc4-f8d773084564