fix modulegroups preset use after free

Author: dterrahe

src/libs/modulegroups.c

@@ -3931,9 +3931,10 @@ void view_enter(dt_lib_module_t *self, dt_view_t *old_view, dt_view_t *new_view)
     dt_lib_modulegroups_t *d = (dt_lib_modulegroups_t *)self->data;
 
     // and we initialize the buttons too
-    const char *preset = dt_conf_get_string_const("plugins/darkroom/modulegroups_preset");
+    char *preset = dt_conf_get_string("plugins/darkroom/modulegroups_preset");
     if(!dt_lib_presets_apply(preset, self->plugin_name, self->version()))
       dt_lib_presets_apply(_(FALLBACK_PRESET_NAME), self->plugin_name, self->version());
+    g_free(preset);
 
     // and set the current group
     d->current = dt_conf_get_int("plugins/darkroom/groups");