lib: posix: Fix pthread_attr_init() return code

pthread_attr_init() should not return EBUSY as per POSIX spec
so fixed this by return ENOMEM if the attr pointer is NULL.

Also fixed the attribute initialization logic by copying the
init_pthread_attrs to the attr.

Fixes Issue #7480

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>