fix degrowth bug

[k47]

r11792 introduced an unwanted behaviour. When trying to build a new house, we are not checking if a given tile is empty. The effect of this is that during build step a random skyscraper is replaced by a level 1 house and city gradually 'degrows'.

[prissi]

Thank you, well spotted. I found the same problem with roads. The best_pos is not the position that is chosen for the build. A few lines below was the right place. r11797 fixes this.

[k47]

I see. I should pay more attention to that big ATTENTION comment above.