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

chergert/gtk,grubersjoe/adwaita,Distrotech/gtk2,grubersjoe/adwaita,jadahl/gtk,jigpu/gtk,jadahl/gtk,jessevdk/gtk,jigpu/gtk,ahodesuka/gtk,ahodesuka/gtk,jessevdk/gtk,jigpu/gtk,bratsche/gtk-,jadahl/gtk,davidt/gtk,davidt/gtk,ebassi/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,chergert/gtk,Distrotech/gtk2,chergert/gtk,ebassi/gtk,Lyude/gtk-,alexlarsson/gtk,msteinert/gtk,msteinert/gtk,jadahl/gtk,jessevdk/gtk,Distrotech/gtk2,grubersjoe/adwaita,Lyude/gtk-,Lyude/gtk-,ebassi/gtk,grubersjoe/adwaita,davidgumberg/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,davidgumberg/gtk,bratsche/gtk-,Sidnioulz/SandboxGtk,davidt/gtk,ebassi/gtk,jigpu/gtk,ahodesuka/gtk,bratsche/gtk-,alexlarsson/gtk,davidgumberg/gtk,davidgumberg/gtk,ahodesuka/gtk,ahodesuka/gtk,alexlarsson/gtk,alexlarsson/gtk,ebassi/gtk,Distrotech/gtk2,Adamovskiy/gtk,ahodesuka/gtk,chergert/gtk,davidgumberg/gtk,ahodesuka/gtk,Adamovskiy/gtk,msteinert/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,Sidnioulz/SandboxGtk,davidgumberg/gtk,jessevdk/gtk,msteinert/gtk,bratsche/gtk-,Adamovskiy/gtk,jigpu/gtk,Lyude/gtk-,jigpu/gtk,Distrotech/gtk2,alexlarsson/gtk,davidt/gtk,bratsche/gtk-,Distrotech/gtk2,Lyude/gtk-,jigpu/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,jessevdk/gtk,Lyude/gtk-,davidgumberg/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,Adamovskiy/gtk,Lyude/gtk-,alexlarsson/gtk,grubersjoe/adwaita,Adamovskiy/gtk,davidt/gtk,jessevdk/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,chergert/gtk,davidt/gtk,alexlarsson/gtk,Lyude/gtk-,jigpu/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jessevdk/gtk,grubersjoe/adwaita,bratsche/gtk-,jadahl/gtk,ebassi/gtk,Adamovskiy/gtk