vim-patch:7.4.369

Problem:    Using freed memory when exiting while compiled with EXITFREE.
Solution:   Set curwin to NULL and check for that. (Dominique Pelle)

https://code.google.com/p/vim/source/detail?r=v7-4-369
Merge pull request #1184 from Shougo/vim-patch-7.4.369

vim-patch:7.4.369