Issue #2379 - Crash on saving

file_save(): make sure we always set an error on failure

file_save_dialog_save_image(): additionally, check that "error" exists
before dereferencing it.

(cherry picked from commit c55f2308e1)

app/dialogs/file-save-dialog.c

@@ -798,7 +798,8 @@ file_save_dialog_save_image (GimpProgress        *progress,
       {
         gimp_message (gimp, G_OBJECT (progress), GIMP_MESSAGE_ERROR,
                       _("Saving '%s' failed:\n\n%s"),
-                      gimp_file_get_utf8_name (file), error->message);
+                      gimp_file_get_utf8_name (file),
+                      error ? error->message : _("Unknown error"));
         g_clear_error (&error);
       }
       break;