[gnome-control-center] printers: Check error parameter is valid

[Marek Kašík <mkasik src gnome org>]

commit 9e0b40b5496e7b17266f3def95b1367ee9f69df9
Author: Robert Ancell <robert ancell canonical com>
Date:   Fri Nov 23 11:04:42 2018 +1300

    printers: Check error parameter is valid

 panels/printers/pp-host.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/panels/printers/pp-host.c b/panels/printers/pp-host.c
index ec5f943fd..cf500d795 100644
--- a/panels/printers/pp-host.c
+++ b/panels/printers/pp-host.c
@@ -554,7 +554,7 @@ pp_host_get_jetdirect_devices_finish (PpHost        *host,
                                       GError       **error)
 {
   g_return_val_if_fail (g_task_is_valid (res, host), NULL);
-
+  g_return_val_if_fail (error == NULL || *error == NULL, NULL);
   return g_task_propagate_pointer (G_TASK (res), error);
 }
 
@@ -764,6 +764,6 @@ pp_host_get_lpd_devices_finish (PpHost        *host,
                                 GError       **error)
 {
   g_return_val_if_fail (g_task_is_valid (res, host), NULL);
-
+  g_return_val_if_fail (error == NULL || *error == NULL, NULL);
   return g_task_propagate_pointer (G_TASK (res), error);
 }