fix test panic

[CaoShuFeng]

Without this change, the following code would panic:

package main

import (
        "fmt"

        jsonpatch "github.com/evanphx/json-patch"
)

func main() {
        original := []byte(`{"name": "John", "age": 24, "height": 3.21}`)
        patchJSON := []byte(`[
                {"op": "test", "path": "/name"}
        ]`)

        patch, err := jsonpatch.DecodePatch(patchJSON)
        if err != nil {
		fmt.Printf("decode %v\n", err)
		return
        }

        modified, err := patch.Apply(original)
        if err != nil {
		fmt.Printf("aplly %v\n", err)
		return
        }

        fmt.Printf("Original document: %s\n", original)
        fmt.Printf("Modified document: %s\n", modified)
}