Fix the issue of null pointer dereference in sb_alloc_per_thread_array

[wu-zhao-min]

• Question: After sb_memalign() returns NULL, the code directly performs memset(ptr, 0, bsize) on the returned pointer without conducting a NULL check.
• Impact: Under conditions of memory shortage, it triggers a segmentation fault, resulting in a Denial of Service (DoS).
• Fix: Add a NULL check before the memset operation.