Fix: Assert from towns looking outside map when trying to build tunnels #8600
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Motivation / Problem
@SamuXarick reported an assertion error during map generation related to GrowTownWithTunnel() checking the slope of a tile outside the map.
Description
When looking up the slope to measure the height of the mountain, the town needs to check the validity of the tile before attempting to measure the slope. Checking a tile outside the map would produce an assertion error in TileHeight() via GetTileSlope(), as in the crash report linked above.
Limitations
I cannot reliably reproduce the assertion error in Scenario Editor or during live gameplay, so testing is based on repeatedly running map generation with and without this change.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.