failed to output.GetRecord(dec)

### summary

[https://github.com/fluent/fluent-bit-go/blob/master/examples/out_multiinstance/out.go#L45](https://github.com/fluent/fluent-bit-go/blob/master/examples/out_multiinstance/out.go#L45)

I have a question about what scenario will fail to execute `output.GetRecord(dec)` ？ 😁 Or, how do I print a decode failure log.

I want to know why it failed. 😁

```go
//export FLBPluginFlushCtx
func FLBPluginFlushCtx(ctx, data unsafe.Pointer, length C.int, tag *C.char) int {
dec := output.NewDecoder(data, int(length))
count := 0
for {
ret, ts, record := output.GetRecord(dec)
if ret != 0 {
break
}
...
}
```

thank u.