fix: potential u64 overflow panic (tari-project#5688)

Description
---
fork_hash_index comes from remote peer, if the remote peer sets this to
u64::Max, the local node will panic.
This data is sanity checked later on, but here a panic is possible

base_layer/core/src/base_node/sync/header_sync/synchronizer.rs

@@ -420,8 +420,25 @@ impl<'a, B: BlockchainBackend + 'static> HeaderSynchronizer<'a, B> {
                     return Err(err.into());
                 },
             };
+            if resp.headers.len() > NUM_INITIAL_HEADERS_TO_REQUEST {
+                self.ban_peer_long(peer, BanReason::PeerSentTooManyHeaders(resp.headers.len()))
+                    .await?;
+                return Err(BlockHeaderSyncError::NotInSync);
+            }
+            if resp.fork_hash_index >= block_hashes.len() as u64 {
+                let _result = self
+                    .ban_peer_long(peer, BanReason::SplitHashGreaterThanHashes {
+                        fork_hash_index: resp.fork_hash_index,
+                        num_block_hashes: block_hashes.len(),
+                    })
+                    .await;
+                return Err(BlockHeaderSyncError::FoundHashIndexOutOfRange(
+                    block_hashes.len() as u64,
+                    resp.fork_hash_index,
+                ));
+            }
 
-            let steps_back = resp.fork_hash_index + offset as u64;
+            let steps_back = resp.fork_hash_index.saturating_add(offset as u64);
             return Ok((resp, block_hashes, steps_back));
         }
     }
@@ -440,14 +457,6 @@ impl<'a, B: BlockchainBackend + 'static> HeaderSynchronizer<'a, B> {
         let (resp, block_hashes, steps_back) = self
             .find_chain_split(sync_peer.node_id(), client, NUM_INITIAL_HEADERS_TO_REQUEST as u64)
             .await?;
-        if resp.headers.len() > NUM_INITIAL_HEADERS_TO_REQUEST {
-            self.ban_peer_long(
-                sync_peer.node_id(),
-                BanReason::PeerSentTooManyHeaders(resp.headers.len()),
-            )
-            .await?;
-            return Err(BlockHeaderSyncError::NotInSync);
-        }
         let proto::FindChainSplitResponse {
             headers,
             fork_hash_index,
@@ -464,19 +473,6 @@ impl<'a, B: BlockchainBackend + 'static> HeaderSynchronizer<'a, B> {
             );
         }
 
-        if fork_hash_index >= block_hashes.len() as u64 {
-            let _result = self
-                .ban_peer_long(sync_peer.node_id(), BanReason::SplitHashGreaterThanHashes {
-                    fork_hash_index,
-                    num_block_hashes: block_hashes.len(),
-                })
-                .await;
-            return Err(BlockHeaderSyncError::FoundHashIndexOutOfRange(
-                block_hashes.len() as u64,
-                fork_hash_index,
-            ));
-        }
-
         // If the peer returned no new headers, this means header sync is done.
         if headers.is_empty() {
             if fork_hash_index > 0 {