Return FALSE instead of NULL, to match the gboolean return type.

Submitted upstream:

  <https://gitlab.gnome.org/GNOME/totem-pl-parser/-/merge_requests/53>

diff --git a/plparse/totem-pl-parser.c b/plparse/totem-pl-parser.c
index 404deb395a229e53..5b9bcf301e2e2c4b 100644
--- a/plparse/totem-pl-parser.c
+++ b/plparse/totem-pl-parser.c
@@ -1334,7 +1334,7 @@ totem_pl_parser_save_finish (TotemPlParser   *parser,
 			     GAsyncResult    *async_result,
 			     GError         **error)
 {
-	g_return_val_if_fail (g_task_is_valid (async_result, parser), NULL);
+	g_return_val_if_fail (g_task_is_valid (async_result, parser), FALSE);
 
 	return g_task_propagate_boolean (G_TASK (async_result), error);
 }