Race condition in entry.Data on calling WithFields() method 

```
fatal error: concurrent map iteration and map write
runtime.mapiternext(0xc000bbee98)
	/usr/local/go/src/runtime/map.go:858 +0x579 fp=0xc000bbedd8 sp=0xc000bbed50 pc=0x1011289
vendor/github.com/sirupsen/logrus.(*Entry).WithFields(0xc0001065b0, 0xc0009d3f50, 0xc0009d3f50)
	vendor/github.com/sirupsen/logrus/entry.go:117 +0xef fp=0xc000bbef08 sp=0xc000bbedd8 pc=0x1479b0f```
