Fix #6389.

[Pentarctagon]

Previously this code assumed that all add-ons would have a _server.pbl or an _info.cfg. When neither was the case, it tried to dereference the begin() of an empty set, causing the crash.

[Pentarctagon]

Tested locally and it's a pretty simple change, so I'll merge this tomorrow if there's no comments.

[Vultraz]

Suggest making remote_version an optional<remote_version>.

[CelticMinstrel]

That would require checking everywhere it's accessed, though, so it would be potentially a much bigger change.

[Vultraz]

[Pentarctagon]

What would that accomplish though? Making remote_version an optional would mean I'd still need to make the same check here (to avoid the bad dereference) while also adding handling anywhere it's currently used rather than setting a default value here once.