Freeciv

On turn change I got a lot of:
in is_tile_rel_req_active() [../../src/common/requirements.c::5094]: assertion 'FALSE' failed.
Illegal value -1 for tile relationship requirement.
Please report this message at https://redmine.freeciv.org/projects/freeciv

This is a custom ruleset (close to variant2). There's also some local code changes, but I find it unlikely (but not impossible) that those are at fault here.

I'll try to come up with reproducible case.

I already noticed that is_tile_rel_req_active() does, at least seemingly, some "breaks;" from the switch -case instead of returning in every case, meaning it might fall to this fc_assert(FALSE) after the switch. But that doesn't explain why requirement value is -1.