unit names with _{a-f}{a-f} in them cause dbus to crash

Fun and profit.

go program to reproduce:
```go
package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"strings"

	"github.com/coreos/go-systemd/dbus"
)

const contents = `[Service]
ExecStart=/bin/true
`

func log(f string, v ...interface{}) {
	msg := fmt.Sprintf(f, v...)
	if !strings.HasSuffix(msg, "\n") {
		msg += "\n"
	}
	fmt.Printf(msg)
}
func die(f string, v ...interface{}) {
	fmt.Fprintf(os.Stderr, f, v...)
	os.Exit(1)
}

func main() {
	if len(os.Args) != 2 {
		die("usage: %s UNIT_NAME\n", os.Args[0])
	}
	name := os.Args[1]
	path := "/tmp/" + name
	sd, err := dbus.New()
	if err != nil {
		die("error initializing dbus connection: %v\n", err)
	} else {
		log("initialized dbus connection")
	}

	if err := ioutil.WriteFile(path, []byte(contents), os.FileMode(0644)); err != nil {
		die("error writing unit: %v\n", err)
	} else {
		log("wrote unit to disk")
	}

	if _, err = sd.LinkUnitFiles([]string{path}, true, true); err != nil {
		die("error linking unit file: %v\n", err)
	} else {
		log("linked runtime unit file")
	}

	if err := sd.Reload(); err != nil {
		die("error reloading systemd: %v\n", err)
	} else {
		log("reloaded systemd")
	}

	if info, err := sd.GetUnitProperties(name); err != nil {
		die("error getting unit properties: %v\n", err)
	} else {
		log("unit properties: %v\n", info)
	}
	os.Exit(0)
}
```

script to test cases:
```bash
#!/bin/bash

trap "exit" INT

pkill dbus-daemon
for i in - _ a b c d e f g h i j k l m n o p q r s t u w x y z; do 
  for j in - _ a b c d e f g h i j k l m n o p q r s t u w x y z; do 
     name=_${i}${j}.service
     ./go-systemd ${name} &> /dev/null
     if [ $? -ne 0 ]; then
       echo "$name failed!"
       pkill dbus-daemon
       sleep 0.1
     fi
  done
done
```

results:
```
core-01 core # ./break.sh 
_aa.service failed!
_ab.service failed!
_ac.service failed!
_ad.service failed!
_ae.service failed!
_af.service failed!
_ba.service failed!
_bb.service failed!
_bc.service failed!
_bd.service failed!
_be.service failed!
_bf.service failed!
_ca.service failed!
_cb.service failed!
_cc.service failed!
_cd.service failed!
_ce.service failed!
_cf.service failed!
_da.service failed!
_db.service failed!
_dc.service failed!
_dd.service failed!
_de.service failed!
_df.service failed!
_ea.service failed!
_eb.service failed!
_ec.service failed!
_ed.service failed!
_ee.service failed!
_ef.service failed!
_fa.service failed!
_fb.service failed!
_fc.service failed!
_fd.service failed!
_fe.service failed!
_ff.service failed!
```