Only modify guioptions if guioptions exists

[EESchneider]

Since v0.11.0, modifying guioptions causes an error in Neovim. In particular, whenever multiple tabs are open, buftabline#update triggers an error message like

Error detected while processing BufAdd Autocommands for "*"..function buftabline#update:
line 2:
E519: Option not supported: guioptions+=e
Press ENTER or type command to continue```

[soyuka]

this deserves to be merged :D

[bufdev]

Merge it!

[ap]

Does this patch also fix it?

diff --git i/plugin/buftabline.vim w/plugin/buftabline.vim
--- i/plugin/buftabline.vim
+++ w/plugin/buftabline.vim
@@ -170,8 +170,10 @@ endfunction

 function! buftabline#update(zombie)
        set tabline=
-       if tabpagenr('$') > 1 | set guioptions+=e showtabline=2 | return | endif
-       set guioptions-=e
+       " set! is used here because neovim lacks guioptions and v0.11.0 broke backcompat
+       " by making setting it an error instead of a no-op
+       if tabpagenr('$') > 1 | set! guioptions+=e showtabline=2 | return | endif
+       set! guioptions-=e
        if 0 == g:buftabline_show
                set showtabline=1
                return