Avoid access beyond memory bounds in pthread_attr_getaffinity_np

Avoid access beyond memory bounds in pthread_attr_getaffinity_np

Resolves BZ #15618.

pthread_attr_getaffinity_np may write beyond bounds of the input
cpuset buffer if the size of the input buffer is smaller than the
buffer present in the input pthread attributes.  Fix is to copy to the
extent of the minimum of the source and the destination.