With this change we will check if the provided buffer number is valid,
before querying its `buftype` option. This is necessary, because
currently we would fail with:
```
Error executing vim.schedule lua callback:
...scope.nvim/lua/telescope/previewers/buffer_previewer.lua:473:
Invalid buffer id: X
```
error, if we try to resume a jumplist picker after doing `:bwipeout`.
4522d7e3ea