lightningd: don't increment plugin state to NEEDS_INIT when error in getmanifest

Otherwise we hangs forever in startup when it was the last plugin, we would
miss destroy_plugin --> check_plugins_manifests --> io_break

e.g. when a plugin tries to register a bool option with a string as default value.

lightningd/plugin.c

@@ -1521,6 +1521,7 @@ static const char *plugin_parse_getmanifest_response(const char *buffer,
 	if (!err)
 		err = plugin_add_params(plugin);
 
+	if (!err)
 		plugin->plugin_state = NEEDS_INIT;
 	return err;
 }