Problem: null pointer member access when closing the only non-float in the
current tab page if autocommands after closing all floats also close all other
tab pages. (making it the last window)
Solution: check last_window again after closing the floats.
Also reduce the scope of "wp"; it would be bugprone to use it before it's later
reassigned to the rv of win_free_mem if freed by Buf/WinLeave.
c14de47f1a