Panic occurs when storage.type filesystem

I got go panic when I set `storage.type filesystem` on fluent bit v1.4.2 and [fluent bit go s3](https://github.com/cosmo0920/fluent-bit-go-s3) v0.7.0.

```
panic: reflect: call of reflect.Value.Index on int64 Value

goroutine 17 [running, locked to thread]:
reflect.Value.Index(0x7f1a281177e0, 0xc0002fb0c0, 0x86, 0x0, 0x0, 0xcd8, 0x1)
/usr/local/go/src/reflect/value.go:956 +0x1b1
github.com/cosmo0920/fluent-bit-go-s3/vendor/github.com/fluent/fluent-bit-go/output.GetRecord(0xc000302b30, 0x3, 0x3, 0xc0000ccd80, 0xcd8)
/go/src/github.com/cosmo0920/fluent-bit-go-s3/vendor/github.com/fluent/fluent-bit-go/output/decoder.go:80 +0x106
main.(*fluentPlugin).GetRecord(0x7f1a287fbda0, 0xc000302b30, 0x627, 0xc0005cae00, 0x6b0, 0x7f1a27ea4029)
/go/src/github.com/cosmo0920/fluent-bit-go-s3/out_s3.go:68 +0x2d
main.FLBPluginFlushCtx(0x0, 0x7f1a26ae10b2, 0x7f1900000f4e, 0x7f1a28d35460, 0x28)
/go/src/github.com/cosmo0920/fluent-bit-go-s3/out_s3.go:306 +0x109
main._cgoexpwrap_6036375e9d10_FLBPluginFlushCtx(0x0, 0x7f1a26ae10b2, 0xf4e, 0x7f1a28d35460, 0x0)
_cgo_gotypes.go:88 +0x49
```

This doesn't always happen and I don't know how to reproduce it.
It seems to have a high probability of occurring when the fluent bit is repeatedly restarted.