DeskClock: Fix NullPointerException in AlarmClockFragment

RingtoneManager.getRingtone could return null so add a check for that

Change-Id: Ic3ec042e7c3fe02eee6388a469f19624d90fdddd
Merge branch 'cm-11.0' of github.com:CyanogenMod/android_packages_apps_DeskClock into kitkat