core: fix possible double-free

If there was data waiting, the task freed it, but then the port
got closed, and the data callback might have freed the data again.

src/mm-plugin-base.c

@@ -370,8 +370,6 @@ supports_task_dispose (GObject *object)
     g_object_unref (priv->port);
     g_free (priv->physdev_path);
     g_free (priv->driver);
-    g_free (priv->probe_resp);
-    g_clear_error (&(priv->probe_error));
 
     for (iter = priv->custom; iter; iter = g_slist_next (iter)) {
         CustomInit *custom = iter->data;
@@ -399,6 +397,8 @@ supports_task_dispose (GObject *object)
 
     g_free (priv->probed_vendor);
     g_free (priv->probed_product);
+    g_free (priv->probe_resp);
+    g_clear_error (&(priv->probe_error));
 
     G_OBJECT_CLASS (mm_plugin_base_supports_task_parent_class)->dispose (object);
 }