fast-export: don't segfault when marks file cannot be opened

The error function only prints an error message, resulting in a
segfault if we later on try to fprintf to a NULL handle.

Fix this by using die_errno instead.

Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fast-export.c

@@ -503,7 +503,7 @@ static void export_marks(char *file)
 
 	f = fopen(file, "w");
 	if (!f)
-		error("Unable to open marks file %s for writing.", file);
+		die_errno("Unable to open marks file %s for writing.", file);
 
 	for (i = 0; i < idnums.size; i++) {
 		if (deco->base && deco->base->type == 1) {