printers: Check whether we have a device name

Check whether we've been able to find a usable name of given device
to avoid dereference of NULL in canonicalize_device_name().

https://bugzilla.gnome.org/show_bug.cgi?id=749896

panels/printers/pp-utils.c

@@ -4161,6 +4161,9 @@ canonicalize_device_name (GList         *devices,
       name = g_strdup (device->device_info);
     }
 
+  if (name == NULL)
+    return NULL;
+
   g_strstrip (name);
   g_strcanon (name, ALLOWED_CHARACTERS, '-');