json2wal: increase reader's buffer size (#3147)

```
panic: failed to unmarshal json: unexpected end of JSON input

goroutine 1 [running]:
main.main()
	/root/gelgo/src/github.com/tendermint/tendermint/scripts/json2wal/main.go:66 +0x73f
```

Closes #3146

scripts/json2wal/main.go

@@ -45,7 +45,10 @@ func main() {
 	}
 	defer walFile.Close()
 
-	br := bufio.NewReader(f)
+	// the length of tendermint/wal/MsgInfo in the wal.json may exceed the defaultBufSize(4096) of bufio
+	// because of the byte array in BlockPart
+	// leading to unmarshal error: unexpected end of JSON input
+	br := bufio.NewReaderSize(f, 2*types.BlockPartSizeBytes)
 	dec := cs.NewWALEncoder(walFile)
 
 	for {