[gnome-software] trivial: Fix a crash on destroy when *not* using the dummy plugin

[Richard Hughes <rhughes src gnome org>]

commit 0d3371aa57b3f8eb0f16fbf2330261c999580174
Author: Richard Hughes <richard hughsie com>
Date:   Fri Sep 9 09:09:02 2016 +0100

    trivial: Fix a crash on destroy when *not* using the dummy plugin

 src/plugins/gs-plugin-dummy.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/plugins/gs-plugin-dummy.c b/src/plugins/gs-plugin-dummy.c
index be5655d..dfcb43f 100644
--- a/src/plugins/gs-plugin-dummy.c
+++ b/src/plugins/gs-plugin-dummy.c
@@ -79,7 +79,8 @@ gs_plugin_destroy (GsPlugin *plugin)
        GsPluginData *priv = gs_plugin_get_data (plugin);
        if (priv->quirk_id > 0)
                g_source_remove (priv->quirk_id);
-       g_object_unref (priv->cached_origin);
+       if (priv->cached_origin != NULL)
+               g_object_unref (priv->cached_origin);
 }
 
 void