Fix a null pointer exception. Add rest routable in link discovery only if the restApi is not null.

src/main/java/net/floodlightcontroller/linkdiscovery/internal/LinkDiscoveryManager.java

@@ -1875,7 +1875,8 @@ IFloodlightModule, IInfoProvider, IHAListener {
         floodlightProvider.addOFSwitchListener(this);
         floodlightProvider.addHAListener(this);
         floodlightProvider.addInfoProvider("summary", this);
-        restApi.addRestletRoutable(new LinkDiscoveryWebRoutable());
+        if (restApi != null)
+            restApi.addRestletRoutable(new LinkDiscoveryWebRoutable());
         setControllerTLV();
     }