Resolves the following error reported in #7949:
```
I0103 21:38:30.230662 s3.go:275 Starting S3 API Server with standard IAM
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x38ca961]
goroutine 102 [running]:
github.com/seaweedfs/seaweedfs/weed/command.(*S3Options).startS3Server(0x7caf840)
/go/src/github.com/seaweedfs/seaweedfs/weed/command/s3.go:295 +0x741
github.com/seaweedfs/seaweedfs/weed/command.runFiler.func1(...)
/go/src/github.com/seaweedfs/seaweedfs/weed/command/filer.go:244
created by github.com/seaweedfs/seaweedfs/weed/command.runFiler in goroutine 1
/go/src/github.com/seaweedfs/seaweedfs/weed/command/filer.go:242 +0x353
```