Add guards for zero gameCount to prevent U256 underflow

lib/base-client/src/lib.rs

@@ -21,7 +21,7 @@ pub async fn finalized_l2_block_number_of_l1_block_number(
         l1_provider,
         l1_block_number,
         l1_dispute_game_factory_proxy,
-        count - U256::ONE,
+        count.saturating_sub(U256::ONE),
     )
     .await
 }

lib/bob-client/src/lib.rs

@@ -21,7 +21,7 @@ pub async fn finalized_l2_block_number_of_l1_block_number(
         l1_provider,
         l1_block_number,
         l1_dispute_game_factory_proxy,
-        count - U256::ONE,
+        count.saturating_sub(U256::ONE),
     )
     .await
 }

voyager/plugins/client-update/base/src/main.rs

@@ -618,6 +618,14 @@ impl Module {
         .await
         .map_err(|err| ErrorObject::owned(-1, ErrorReporter(err).to_string(), None::<()>))?;
 
+        if game_index.is_zero() {
+            return Err(ErrorObject::owned(
+                -1,
+                "no dispute games at this L1 block height".to_string(),
+                None::<()>,
+            ));
+        }
+
         let game_index = game_index - U256::ONE;
 
         let dispute_game_factory_account_proof = self

voyager/plugins/client-update/bob/src/main.rs

@@ -630,6 +630,14 @@ impl Module {
         .await
         .map_err(|err| ErrorObject::owned(-1, ErrorReporter(err).to_string(), None::<()>))?;
 
+        if game_index.is_zero() {
+            return Err(ErrorObject::owned(
+                -1,
+                "no dispute games at this L1 block height".to_string(),
+                None::<()>,
+            ));
+        }
+
         let game_index = game_index - U256::ONE;
 
         let dispute_game_factory_account_proof = self