If link list if null or empty, the following code links.get(0) will get an error.

Change-Id: I51c1cdc374f1b550e942826fb65acb40161f7d86
diff --git a/apps/segmentrouting/app/src/main/java/org/onosproject/segmentrouting/mcast/McastHandler.java b/apps/segmentrouting/app/src/main/java/org/onosproject/segmentrouting/mcast/McastHandler.java
index f359998..e90de03 100644
--- a/apps/segmentrouting/app/src/main/java/org/onosproject/segmentrouting/mcast/McastHandler.java
+++ b/apps/segmentrouting/app/src/main/java/org/onosproject/segmentrouting/mcast/McastHandler.java
@@ -1281,6 +1281,10 @@
 
     private void installPath(IpAddress mcastIp, ConnectPoint source, Path mcastPath) {
         List<Link> links = mcastPath.links();
+        if (links.isEmpty()) {
+            log.warn("There is no link that can be used. Stopping installation.");
+            return;
+        }
         // Setup new ingress mcast role
         mcastRoleStore.put(new McastRoleStoreKey(mcastIp, links.get(0).src().deviceId(), source),
                            INGRESS);