printing: Fail nicely when /tmp is not writable

Don't crash when /tmp is not writable when printing to file.
Show that getting of printer details failed for CUPS printers.

https://bugzilla.gnome.org/show_bug.cgi?id=693200

modules/printbackends/cups/gtkprintbackendcups.c

@@ -3489,7 +3489,7 @@ cups_request_ppd (GtkPrinter *printer)
   GtkPrintBackend *print_backend;
   GtkPrinterCups *cups_printer;
   GtkCupsRequest *request;
-  char *ppd_filename;
+  char *ppd_filename = NULL;
   gchar *resource;
   http_t *http;
   GetPPDData *data;