syscalls.master: Fix SAL annotation for getdirentires basep argument

getdirentires last argument "off_t *basep" is an optional output
argument. It returns the value only when the passed-in value(pointer)
is non-NULL.

This is a part of the research work at RCSLab, University of Waterloo.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D41969