gkeyfilesettingsbackend: Add assertion for known-true condition

We know the group exists since we enumerated it.  Should silence a
static analysis warning.

https://bugzilla.gnome.org/show_bug.cgi?id=733576

gio/gkeyfilesettingsbackend.c

@@ -392,6 +392,7 @@ keyfile_to_tree (GKeyfileSettingsBackend *kfsb,
         continue;
 
       keys = g_key_file_get_keys (keyfile, groups[i], NULL, NULL);
+      g_assert (keys != NULL);
 
       for (j = 0; keys[j]; j++)
         {