Parcel: Avoid NPE in finalization

Check whether the guard is null to avoid:

 Uncaught exception thrown by finalizer
 java.lang.NullPointerException: Attempt to invoke virtual method 'void dalvik.system.CloseGuard.close()' on a null object reference
      at android.os.ParcelFileDescriptor.closeWithStatus(ParcelFileDescriptor.java:740)
      at android.os.ParcelFileDescriptor.finalize(ParcelFileDescriptor.java:990)
      at java.lang.Daemons$FinalizerDaemon.doFinalize(Daemons.java:250)
      at java.lang.Daemons$FinalizerDaemon.runInternal(Daemons.java:237)
      at java.lang.Daemons$Daemon.run(Daemons.java:103)
      at java.lang.Thread.run(Thread.java:764)

Follow-up to commit da5a3e12.

Bug: 7426029
Bug: 10330121
Test: m
Change-Id: I903f1545ab784008727ff23bb95fe182bd95b62a