spice: do not call get_pixbuf() if the display is not ready
spice: do not call get_pixbuf() if the display is not ready

Avoid the infamous d->data != NULL critical when the display is not
ready. Sadly, get_pixbuf() doesn't raise errors, so it makes sense to
not call the function when the display is not ready.

https://bugzilla.gnome.org/show_bug.cgi?id=683274

vbenes/gnome-boxes,vbenes/gnome-boxes