[commit/obvious] guard against NULL in TUI code

[Thiago Jung Bauermann]

Hi,

I just committed this patch, which fixes a segfault that can be
triggered by:

$ gdb
(gdb) focus cmd
(gdb) focus prev

I just added a NULL check to tui_prev_win which was already present in
the corresponding tui_next_win function.
-- 
[]'s
Thiago Jung Bauermann
Software Engineer
IBM Linux Technology Center
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