[bug] Wall/Rampart building code gets stuck on impossible to built positions

[tedivm]

This is primarily an issue because of another already solved bug, but it should still be resolved for robustness.

Basically, if a rampart or wall is scheduled to be built but it is in an impossible to build on position it will attempt to add the construction site and fail. The next tick it will see that there is no site there and will try again.

This can be resolved by adding a check when looking for "gaps" to see if it is possible to construct and then skipping over those that aren't.