Fix panic on getting safe block

Since bor doesn't have "safe" block, the api should always return null when requested safe block.

eth/api_backend.go

@@ -157,7 +157,11 @@ func (b *EthAPIBackend) BlockByNumber(ctx context.Context, number rpc.BlockNumbe
 	if number == rpc.SafeBlockNumber {
 		header := b.eth.blockchain.CurrentSafeBlock()
 
-		return b.eth.blockchain.GetBlock(header.Hash(), header.Number.Uint64()), nil
+		if header == nil {
+			return nil, errors.New("safe block not found")
+		} else {
+			return b.eth.blockchain.GetBlock(header.Hash(), header.Number.Uint64()), nil
+		}
 	}
 
 	return b.eth.blockchain.GetBlockByNumber(uint64(number)), nil

internal/ethapi/api_test.go

@@ -1818,6 +1818,11 @@ func TestRPCGetBlockOrHeader(t *testing.T) {
 			fullTx:    true,
 			file:      "hash-pending-fullTx",
 		},
+		// 26. safe block
+		{
+			blockNumber: rpc.SafeBlockNumber,
+			file:        "tag-safe",
+		},
 	}
 
 	for i, tt := range testSuite {

internal/ethapi/testdata/eth_getBlockByNumber-tag-safe.json

@@ -0,0 +1 @@
+null