It's possible that the mac address is null (getMacAddress() of WifiNative.java).
In this case system_server will crash like:
E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: WifiWatchdogStateMachine
E AndroidRuntime: java.lang.NullPointerException
E AndroidRuntime: at android.net.arp.ArpPeer.<init>(ArpPeer.java:57)
E AndroidRuntime: at android.net.wifi.WifiWatchdogStateMachine.doArpTest(WifiWatchdogStateMachine.java:866)
Check the mac address before parsing to avoid crashing.
Change-Id: I5d4205c04d479a3a2837172c6382816ea4bf74d6