Fix #11801, 51f1e93: CalcClosestTownFromTile needs the kd-tree to be valid

(cherry picked from commit 4cc97e04e6)

src/sl/oldloader_sl.cpp

@@ -497,7 +497,12 @@ static Town *RemapTown(TileIndex fallback)
 {
 	/* In some cases depots, industries and stations could refer to a missing town. */
 	Town *t = Town::GetIfValid(RemapTownIndex(_old_town_index));
-	if (t == nullptr) t = CalcClosestTownFromTile(fallback);
+	if (t == nullptr) {
+		/* In case the town that was refered to does not exist, find the closest.
+		 * However, this needs the kd-tree to be present. */
+		RebuildTownKdtree();
+		t = CalcClosestTownFromTile(fallback);
+	}
 	return t;
 }