backup: Fix the system server crash fix

We still need the firefighters after commit eff323bf.

--------- beginning of crash
E/AndroidRuntime(649): *** FATAL EXCEPTION IN SYSTEM PROCESS: backup
E/AndroidRuntime(649): java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.FileDescriptor android.os.ParcelFileDescriptor.getFileDescriptor()' on a null object reference
E/AndroidRuntime(649):       at com.android.server.backup.BackupManagerService$PerformBackupTask.operationComplete(BackupManagerService.java:2826)
E/AndroidRuntime(649):       at com.android.server.backup.BackupManagerService$BackupHandler.handleMessage(BackupManagerService.java:778)
E/AndroidRuntime(649):       at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime(649):       at android.os.Looper.loop(Looper.java:135)
E/AndroidRuntime(649):       at android.os.HandlerThread.run(HandlerThread.java:61)

Change-Id: I28a9e8920bb33e687394fd6da6daede31bd77420