Avoid OOB reads in memmem(3).

commit 51bdcdc424bd7169c8cccdc2de7cad17f5ea0f70
Author: Alexander Monakov <amonakov@ispras.ru>
Date: Fri Jun 30 00:35:33 2017 +0300

fix OOB reads in Xbyte_memmem

Reported by Leah Neukirchen.

Reviewed by: emaste
Approved by: re (kib)