fix bad tile crash (#1237)

## Description:

Sending invalid coords can cause game to crash. Make sure to validate
tile ref.

## 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:
evan

src/core/execution/ExecutionManager.ts

@@ -1,5 +1,4 @@
 import { Execution, Game } from "../game/Game";
-import { TileRef } from "../game/GameMap";
 import { PseudoRandom } from "../PseudoRandom";
 import { ClientID, GameID, Intent, Turn } from "../Schemas";
 import { simpleHash } from "../Util";
@@ -72,16 +71,12 @@ export class Executor {
           this.mg.ref(intent.x, intent.y),
         );
       case "boat":
-        let src: TileRef | null = null;
-        if (intent.srcX !== null && intent.srcY !== null) {
-          src = this.mg.ref(intent.srcX, intent.srcY);
-        }
         return new TransportShipExecution(
           player,
           intent.targetID,
-          this.mg.ref(intent.dstX, intent.dstY),
+          intent.dst,
           intent.troops,
-          src,
+          intent.src,
         );
       case "allianceRequest":
         return new AllianceRequestExecution(player, intent.recipient);