[file-roller] fixed message dialog showing a (null) string

[Paolo Bacchilega <paobac src gnome org>]

commit aea2a9453972c7e933a7de5254abb5a9d35e2cb9
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Sun Dec 13 10:29:24 2015 +0100

    fixed message dialog showing a (null) string

 src/gtk-utils.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/gtk-utils.c b/src/gtk-utils.c
index e1e1ab9..3bce41c 100644
--- a/src/gtk-utils.c
+++ b/src/gtk-utils.c
@@ -70,7 +70,8 @@ _gtk_message_dialog_new (GtkWindow      *parent,
                                         GTK_BUTTONS_NONE,
                                         "%s", message);
 
-       gtk_message_dialog_format_secondary_text (GTK_MESSAGE_DIALOG (dialog), "%s", secondary_message);
+       if (secondary_message != NULL)
+               gtk_message_dialog_format_secondary_text (GTK_MESSAGE_DIALOG (dialog), "%s", 
secondary_message);
 
        if (flags & GTK_DIALOG_MODAL)
                _gtk_dialog_add_to_window_group (GTK_DIALOG (dialog));