Ghost users crash buffers

[wd60622]

Example Issue: pymc-devs/nutpie#115

Error executing vim.schedule lua callback: .../wdean/GitHub/neovim-plugins/octo.nvim/lua/octo/init.lua:94: Err
or executing lua: .../neovim-plugins/octo.nvim/lua/octo/model/octo-buffer.lua:51: attempt to index field 'auth
or' (a userdata value)
stack traceback:
        .../neovim-plugins/octo.nvim/lua/octo/model/octo-buffer.lua:51: in function 'new'
        .../wdean/GitHub/neovim-plugins/octo.nvim/lua/octo/init.lua:261: in function 'create_buffer'
        .../wdean/GitHub/neovim-plugins/octo.nvim/lua/octo/init.lua:95: in function <.../wdean/GitHub/neovim-p
lugins/octo.nvim/lua/octo/init.lua:94>
        [C]: in function 'nvim_buf_call'
        .../wdean/GitHub/neovim-plugins/octo.nvim/lua/octo/init.lua:94: in function 'cb'
        .../wdean/GitHub/neovim-plugins/octo.nvim/lua/octo/init.lua:133: in function 'cb'
        ...ean/GitHub/neovim-plugins/octo.nvim/lua/octo/gh/init.lua:163: in function ''
        vim/_editor.lua: in function <vim/_editor.lua:0>
stack traceback:
        [C]: in function 'nvim_buf_call'
        .../wdean/GitHub/neovim-plugins/octo.nvim/lua/octo/init.lua:94: in function 'cb'
        .../wdean/GitHub/neovim-plugins/octo.nvim/lua/octo/init.lua:133: in function 'cb'
        ...ean/GitHub/neovim-plugins/octo.nvim/lua/octo/gh/init.lua:163: in function ''
        vim/_editor.lua: in function <vim/_editor.lua:0>