Can get opponent last fire location

rust/sea_battle_backend/src/data/boats_layout.rs

@@ -85,6 +85,10 @@ impl Coordinates {
         }
     }
 
+    pub fn invalid() -> Self {
+        Self { x: -1, y: -1 }
+    }
+
     pub fn is_valid(&self, rules: &GameRules) -> bool {
         self.x >= 0
             && self.y >= 0