Segfault while rendering a road

While playing against a bunch of AIs, I got a segfault. I suspect that the segfault happens in RoadProgram::add_road, l.78.

When debugging with gdb, the assert(start.owner != nullptr) in l.75 fails before, so we're not getting to the actual segfault. There was a battle happening a few minutes before the bug, maybe that has something to do withe the field having no owner.

To reproduce, load the attached savgame and wait a few minutes. The error should ocure around 2:50.