libhtr: pthread_attr_setaffinity_np(): Replace calloc() with malloc()

[olce]

Summary

Using calloc() instead of malloc() is useless here since the allocated
memory is to be wholly crushed by the memcpy() call that follows.

Suggested by: kib
MFC after: 2 weeks

[olce]

The typo in the title ("libhtr" => "libthr") will be fixed on commit.

[emaste]

Approved