Fix signs for diagnostics that are reported one line past the end

[dotdash]

Some language servers report unexpected EOF errors on the line that's actually one past the last line of the file. Be a bit more accomodating and just show them on the last line of the buffer instead.

[dotdash]

I've noticed this problem with the Intelephense LSP server.

[dotdash]

@prabirshrestha the failing test seems to be from an intermittent github issue, could you re-run it please?

[prabirshrestha]

Merged. Thanks.