gutf8: add NULL checking

This fixes nautilus crash and perhaps other callers issues.

Nautilus (and sometimes glib) crashes with malformed uri inside of the bookmarks file .config/gtk-3.0/bookmarks.

Surely it might be a bug in nautilus, since it should sanitize its own inputs.

In despite of that, it's sane not to trust in callers values.

Signed-off-by: Geyslan G. Bem geyslan@gmail.com