Crash with map resize

[Amelek]

1. Create map with huge number of rivers but no water body - no rivers will appear
2. Dig hole to water near border of the map.
3. Fence it with power line
4. Resize map - game will crash with vector template assertion



Some time ago, instead of crashing, game would make a river under power line, but due to impossibility of lowering terrain, it will instead build river that goes up the hill. Crash might be related with fix to this issue.