Fix a potential NULL pointer dereference in _init_thread().

The first NULL pointer check against `attr' suggests that `attr' can
be NULL.  Then later `attr' is directly dereferenced, suggesting the
opposite.

    if (attr == NULL) {
        ...
    } else {
        ...
    }
    ...
    if (attr->stack_base == ...) { ... }

The public API pthread_create(3) allows NULL, and interprets it as "default".
Our implementation actually swaps in a pointer to the global default
pthread_attr_t, so we don't need any NULL checks in _init_thread. (The other
internal caller passes its own pthread_attr_t.)

Change-Id: I0a4e79b83f5989249556a07eed1f2887e96c915e
Signed-off-by: Xi Wang <xi.wang@gmail.com>