anyxml.Xml segfaults on a nil input

[dechamps]

Steps to reproduce:

package main
  
import (
        "fmt"
        "github.com/clbanning/anyxml"
)

func main() {
        xmlout, err := anyxml.Xml(nil, "root")
        if err != nil {
                return
        }
        fmt.Println(string(xmlout))
}

Expected result:

<root />

Actual result:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x98 pc=0x4a1f1a]

goroutine 1 [running]:
github.com/clbanning/anyxml.Xml(0x0, 0x0, 0xc420059f68, 0x1, 0x1, 0xc420059f78, 0x403d8c, 0xc42001a178, 0x0, 0x0)
	/home/e-t172/go/src/github.com/clbanning/anyxml/anyxml.go:57 +0x6a
main.main()
	/home/e-t172/dev/anyxml/nil.go:9 +0x6f
exit status 2

[dechamps]

(See #3 for why this was closed.)