Problem:
If there are errors in the last line of a buffer, something like `Gdk` or
`G2k3J` will produce an error (at least with `lua_ls`):
Error executing vim.schedule lua callback:
.../neovim/share/nvim/runtime/lua/vim/diagnostic.lua:1446: Invalid 'line': out of range
Solution:
Only set extmarks if the target buffer line still exists
(cherry picked from commit e5e81262af)
21157459fe
