libnm-core: fix memory leak in nm_setting_to_string()

Fixes: f957ea2b34 ('core/setting: rework nm_connection_dump()')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1934
(cherry picked from commit ee5f4402bf)

src/libnm-core-impl/nm-setting.c

@@ -3745,6 +3745,7 @@ nm_setting_to_string(NMSetting *setting)
         value_str = g_variant_print(value, FALSE);
 
         g_string_append_printf(string, "\t%s : %s\n", name, value_str);
+        g_variant_unref(child);
     }
 
     return g_string_free(string, FALSE);