Fixed t:undotree undefined error
Fixed t:undotree undefined error

the ```t:undotree.GetStatusLine()``` is appended to the status line when ```t:undotree``` exists, but it seems like the ```statusline``` will be executed when ```t:undtree``` is no longer available during window destruction.

bling/vim-airline,chrisbra/vim-airline,yzhou61/vim-airline,rchicoli/vim-airline