panic nil pointer with v1.10.0

Since last release, I have:

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x731009]
goroutine 1 [running]:
main.main()
	github.com/homeport/dyff@v1.10.0/cmd/dyff/main.go:47 +0x1c9
```

The command line :
` dyff bw -s -i dir1 dir2`