Fix reboot loop.

The multinetwork branch is rebooting on startup with:

E/AndroidRuntime( 1024): *** FATAL EXCEPTION IN SYSTEM PROCESS: NetworkStats
[...]
E/AndroidRuntime( 1024): Caused by: java.lang.NullPointerException: Attempt to read from field 'long android.net.NetworkCapabilities.mNetworkCapabilities' on a null object reference
E/AndroidRuntime( 1024): 	at android.net.NetworkCapabilities.<init>(NetworkCapabilities.java:235)
E/AndroidRuntime( 1024): 	at android.net.BaseNetworkStateTracker.getNetworkCapabilities(BaseNetworkStateTracker.java:103)
E/AndroidRuntime( 1024): 	at com.android.server.ConnectivityService.getAllNetworkState(ConnectivityService.java:1134)
E/AndroidRuntime( 1024): 	at com.android.server.net.NetworkStatsService.updateIfacesLocked(NetworkStatsService.java:877)
E/AndroidRuntime( 1024): 	at com.android.server.net.NetworkStatsService.updateIfaces(NetworkStatsService.java:849)
E/AndroidRuntime( 1024): 	at com.android.server.net.NetworkStatsService.access$600(NetworkStatsService.java:139)
E/AndroidRuntime( 1024): 	at com.android.server.net.NetworkStatsService$2.onReceive(NetworkStatsService.java:717)
E/AndroidRuntime( 1024): 	at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:766)
E/AndroidRuntime( 1024): 	... 4 more

This seems to be due to NetworkCapabilities' copy constructor
not null-checking its argument.  Unbreak things by ignoring null.

Change-Id: Iff3c38e6d72390fa86e51bfce534ebd42a262e19