Fix: check if the buffer is still loaded before calling nvim_buf_attach

[JafarAbdi]

I'm getting the following error when calling bwipeout on a buffer, so this PR fixes it.

Not sure if this's the best solution since we already have another check gitsigns.tl#L209-L212 in the same function.

Error executing vim.schedule lua callback: ...e/pack/packer/start/gitsigns.nvim/lua/gitsigns/async.lua:62: The coroutine failed with this message: ...im/site/pack/packer/start/gitsigns.nvim/lua/gitsigns.lua:294: Invalid buffer id: 5
stack traceback:
        [C]: in function 'nvim_buf_attach'
        ...im/site/pack/packer/start/gitsigns.nvim/lua/gitsigns.lua:294: in function 'fn'
        ...ack/packer/start/gitsigns.nvim/lua/gitsigns/debounce.lua:78: in function 'attach_throttled'
        ...im/site/pack/packer/start/gitsigns.nvim/lua/gitsigns.lua:316: in function <...im/site/pack/packer/start/gitsigns.nvim/lua/gitsigns.lua:315>
stack traceback:
        [C]: in function 'error'
        ...e/pack/packer/start/gitsigns.nvim/lua/gitsigns/async.lua:62: in function <...e/pack/packer/start/gitsigns.nvim/lua/gitsigns/async.lua:57>

• Have you read CONTRIBUTING.md?

[lewis6991]

This can happen because there is async code between the check you mentioned and the buffer attaching.

Might not be the most ideal solution, but we can merge it for now.