printing: Make sure destroy function is always called by gtk_enumerate_printers

printing: Make sure destroy function is always called by gtk_enumerate_printers

When GtkPrinterFunc always returns FALSE, for example when looking for
a non existent printer, if print list is done for all backends or print
backend status is UNAVAILABLE, gtk_enumerate_printers() finishes with an
empty backend list and destroy function is never called. We need to
check the backend list again after calling list_printers_init for all
backends and finish the enumeration if it's empty.

https://bugzilla.gnome.org/show_bug.cgi?id=672125