Disconnected walls don't decay anymore

[elexis]

Reproduce: Start a game with iberian, delete the CC (or another civ and build the walls)

It was not introduced by r17053 (which fixed #3426) as the commit before that also has this problem.

With r16500 unconnected walls were decaying and that should be the case (close as won't fix otherwise).

[Stan]

Duplicate of #3340.

[elexis]

Ah, thx 4 finding that out.

[Stan]

Ticketman to the rescue :D