Do not merge: Add some input checking to prevent problems

Adding nulls can cause unhappiness.  It's suspected as a cause of 3495045.

bug:3495045
Change-Id: I925c965ef64032fc57849bf15fe9b11f4bc99351
diff --git a/core/java/android/net/LinkProperties.java b/core/java/android/net/LinkProperties.java
index b6e9751..81d62a0 100644
--- a/core/java/android/net/LinkProperties.java
+++ b/core/java/android/net/LinkProperties.java
@@ -91,7 +91,7 @@
     }
 
     public void addLinkAddress(LinkAddress address) {
-        mLinkAddresses.add(address);
+        if (address != null) mLinkAddresses.add(address);
     }
 
     public Collection<LinkAddress> getLinkAddresses() {
@@ -99,7 +99,7 @@
     }
 
     public void addDns(InetAddress dns) {
-        mDnses.add(dns);
+        if (dns != null) mDnses.add(dns);
     }
 
     public Collection<InetAddress> getDnses() {
@@ -107,7 +107,7 @@
     }
 
     public void addGateway(InetAddress gateway) {
-        mGateways.add(gateway);
+        if (gateway != null) mGateways.add(gateway);
     }
     public Collection<InetAddress> getGateways() {
         return Collections.unmodifiableCollection(mGateways);