Fix NPE in ConnectivityService.

If it's configured with a defined network but can't create a network state tracker
for it, it would NPE and restart the framework whenever a default network
disconnects.

bug:5603268
Change-Id: I816c4f522d766e0353a713623f6635b03395b01e

Author: Robert Greenwalt

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

@@ -1597,6 +1597,7 @@ private void tryFailover(int prevNetType) {
                 if (checkType == prevNetType) continue;
                 if (mNetConfigs[checkType] == null) continue;
                 if (!mNetConfigs[checkType].isDefault()) continue;
+                if (mNetTrackers[checkType] == null) continue;
 
 // Enabling the isAvailable() optimization caused mobile to not get
 // selected if it was in the middle of error handling. Specifically