Fix Wimax-less build.

A build with the wimax network type defined but wimax disabled
causes an NPE in ConnectivityService's constructor.

bug:5237167
Change-Id: I929eac217e1afa0e61346fdbc3e96a7d3ad09a54

services/java/com/android/server/ConnectivityService.java

@@ -514,7 +514,9 @@ public class ConnectivityService extends IConnectivityManager.Stub {
                 continue;
             }
             mCurrentLinkProperties[netType] = null;
-            if (mNetConfigs[netType].isDefault()) mNetTrackers[netType].reconnect();
+            if (mNetTrackers[netType] != null && mNetConfigs[netType].isDefault()) {
+                mNetTrackers[netType].reconnect();
+            }
         }
 
         IBinder b = ServiceManager.getService(Context.NETWORKMANAGEMENT_SERVICE);