Merge pull request from #775 from hollyshi:patch-2

The operator '+' has a higher operation priority than '?:'. A NPE will be thrown if getUrl() returns null, which is not expected.

Author: cuteason

dubbo-rpc/dubbo-rpc-api/src/main/java/com/alibaba/dubbo/rpc/listener/ListenerInvokerWrapper.java

@@ -76,7 +76,7 @@ public Result invoke(Invocation invocation) throws RpcException {
 
     @Override
     public String toString() {
-        return getInterface() + " -> " + getUrl() == null ? " " : getUrl().toString();
+        return getInterface() + " -> " + (getUrl() == null ? " " : getUrl().toString());
     }
 
     public void destroy() {
@@ -97,4 +97,4 @@ public void destroy() {
         }
     }
 
-}
+}