gio: Fix a GError memory leak in GNetworkService

Spotted by Coverity (CID: #1325405).
diff --git a/gio/gnetworkservice.c b/gio/gnetworkservice.c
index d759819..57c8e8f 100644
--- a/gio/gnetworkservice.c
+++ b/gio/gnetworkservice.c
@@ -480,6 +480,8 @@
             {
               if (srv_enum->error == NULL)
                 srv_enum->error = error;
+              else
+                g_error_free (error);
               continue;
             }