Should ensure valid UTF-8, since CUPS doesn't

[Tim Waugh]

CUPS may provide invalid UTF-8 strings for some job attributes, as they have been accepted from clients unvalidated. This can cause problems when handling those strings, as in this report:
  https://bugzilla.redhat.com/show_bug.cgi?id=871223

Until CUPS validates all client-provided strings, clients will need to perform their own validation for robustness in the face of other clients who provide strings in the wrong encoding.

Here is a patch to do that for systemtray.