Avoid system_server crashing due to mac address is null

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

Author: cwhuang

core/java/android/net/arp/ArpPeer.java

@@ -53,9 +53,11 @@ public ArpPeer(String interfaceName, InetAddress myAddr, String mac,
         mInterfaceName = interfaceName;
         mMyAddr = myAddr;
 
-        for (int i = 0; i < MAC_ADDR_LENGTH; i++) {
-            mMyMac[i] = (byte) Integer.parseInt(mac.substring(
-                        i*3, (i*3) + 2), 16);
+        if (mac != null) {
+            for (int i = 0; i < MAC_ADDR_LENGTH; i++) {
+                mMyMac[i] = (byte) Integer.parseInt(mac.substring(
+                            i*3, (i*3) + 2), 16);
+            }
         }
 
         if (myAddr instanceof Inet6Address || peer instanceof Inet6Address) {