Wall-builder spits an error when there aren't enough resources

[sanderd17]

When you build a wall, but the last pieces turn red due to lack of resources, it throws to following error (once):

ERROR: [TryConstructWall] No entity ID set for constructed entity of template 'structures/rome_wall_tower'

Apart from that, it doesn't have any affect on the gameplay, so it's just mildly annoying and could cause some bug reports when this gets released.

[Itms]

In 17955:

Properly fix the typo mentioned in r17714. The ++i in the for loop already increments i so doing it once more is not wanted.
Fixes #3854.