ofz#817 nStrLen-1 changed to nStrLen-3

regression from...

commit ff8f6629287d0d1bdbcf4f8ed16cb8d077d08c7a
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Thu Jan 19 16:56:34 2017 +0000

    Resolves: ofz#424 guard against broken dxary length

but this weird typo doesn't appears in the 5-2 and 5-3 backports,
odd how I managed that

Change-Id: I5fb1db2284d48ee78e717d41274a3d37ab0255cf