[neovim] Fix error in finally block when callback failed

e.g. Opening another buffer when `set nohidden`

https://github.com/junegunn/fzf.vim/issues/77

plugin/fzf.vim

@@ -344,6 +344,7 @@ function! s:execute_term(dict, command, temps)
     endif
 
     call s:pushd(self.dict)
+    let ret = []
     try
       let ret = s:callback(self.dict, self.temps)