[gnome-software] Add a missing error check to fix a common crash on LiveDVD media

[Richard Hughes <rhughes src gnome org>]

commit ca06f7b82e04d53883282bda898cbf40ebdb7548
Author: Richard Hughes <richard hughsie com>
Date:   Thu Dec 1 20:15:48 2016 +0000

    Add a missing error check to fix a common crash on LiveDVD media
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1393344

 src/plugins/gs-plugin-odrs.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/plugins/gs-plugin-odrs.c b/src/plugins/gs-plugin-odrs.c
index 5e2001e..c519635 100644
--- a/src/plugins/gs-plugin-odrs.c
+++ b/src/plugins/gs-plugin-odrs.c
@@ -182,6 +182,8 @@ gs_plugin_odrs_refresh_ratings (GsPlugin *plugin,
                                          "odrs.json",
                                          GS_UTILS_CACHE_FLAG_WRITEABLE,
                                          error);
+       if (fn == NULL)
+               return FALSE;
        if (cache_age > 0) {
                guint tmp;
                g_autoptr(GFile) file = NULL;