Mandate that inputs are valid UTF8
This restriction was already in place, since we passed the inputs directly to GLib functions that expect valid UTF8.
If this library were widely used then we might proactively validate inputs to protect against exploits. At time of writing, this module is only used in 3 places (grilo-plugins, gnome-music and rygel), so just check they are doing the right thing when calling the API.
See !5 for background.