Fix NPE when going from proxy to no proxy.

bug:5081431
Change-Id: I68a8cca94e4612bd9ee2faf1123e0b63dc25b833

Author: Robert Greenwalt

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

@@ -2299,7 +2299,7 @@ private void handleApplyDefaultProxy(int type) {
         synchronized (this) {
             if (mDefaultProxy != null && mDefaultProxy.equals(proxy)) return;
             if (mDefaultProxy == proxy) return;
-            if (!TextUtils.isEmpty(proxy.getHost())) {
+            if (proxy != null && !TextUtils.isEmpty(proxy.getHost())) {
                 mDefaultProxy = proxy;
             } else {
                 mDefaultProxy = null;