fix net_services_name retval

bindings/ruby/examples/netstat.rb

@@ -14,7 +14,7 @@ def format_port(sigar, proto, port, is_numeric)
   end
   if !is_numeric
     service = sigar.net_services_name(proto, port)
-    if service != false
+    if service != nil
       return service
     end
   end

bindings/ruby/rbsigar.c

@@ -212,7 +212,7 @@ static VALUE rb_sigar_net_services_name(VALUE obj, VALUE protocol, VALUE port)
         return rb_str_new2(name);
     }
     else {
-        Qnil;
+        return Qnil;
     }
 }