[r6949] assertion fails after building elevated road

[whoami]

Try rev.6949 nightly Win-SDL with Pak128.Britain nightly (r1338): build a short section of an elevated road on the default map. Quit the program -> "assertion failed" appears (at least for me). (Stacking the elevated ways is not necessary.) Save, exit, restart, reload seems to work in my test cases, so no harm has been observed yet.

[Dwachs]

should be fixed

[whoami]

Yes, I cannot reproduce it any more with r6960.