helper_thread: check for null scalloc return value

helper_thread: check for null scalloc return value

scalloc can return NULL if it fails to allocate memory. Check for this
condition and fail the helper thread setup if it occurs.

This issue was reported by Coverity:

** CID 496644:  Null pointer dereferences  (NULL_RETURNS)
/helper_thread.c: 425 in helper_thread_create()
419
420     	hd = scalloc(1, sizeof(*hd));
421
422     	setup_disk_util();
423     	steadystate_setup();
424
>>>     CID 496644:  Null pointer dereferences  (NULL_RETURNS)
>>>     Dereferencing "hd", which is known to be "NULL".
425     	hd->sk_out = sk_out;

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>