fix is valid ref (#1240)

## Description:

Fix the isValidRef check. Calling x() or y() on invalid ref causes it to
throw exception.
## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

DISCORD_USERNAME

src/core/game/GameMap.ts

@@ -118,7 +118,7 @@ export class GameMapImpl implements GameMap {
   }
 
   isValidRef(ref: TileRef): boolean {
-    return this.isValidCoord(this.x(ref), this.y(ref));
+    return ref >= 0 && ref < this.refToX.length;
   }
 
   x(ref: TileRef): number {