Prevent NPE in new API.

bug:8455284
Change-Id: I26ecbd024d1fe001f3af792c4e66409b2968c7ec
diff --git a/core/java/android/net/LinkProperties.java b/core/java/android/net/LinkProperties.java
index e522754..eedc372 100644
--- a/core/java/android/net/LinkProperties.java
+++ b/core/java/android/net/LinkProperties.java
@@ -121,7 +121,7 @@
 
     public Collection<String> getAllInterfaceNames() {
         Collection interfaceNames = new ArrayList<String>(mStackedLinks.size() + 1);
-        interfaceNames.add(new String(mIfaceName));
+        if (mIfaceName != null) interfaceNames.add(new String(mIfaceName));
         for (LinkProperties stacked: mStackedLinks.values()) {
             interfaceNames.addAll(stacked.getAllInterfaceNames());
         }