heap-buffer-overflow in pthreadpool_create()

see https://issuetracker.google.com/211696118 for the original bug report.

the code's moved about and is at https://github.com/Maratyszcza/pthreadpool/blob/a134dd5d4cee80cce15db81a72e7f929d71dd413/src/memory.c#L27 now, but the bug's still there.

probably wants to be something like this instead:

	const size_t threadpool_size;
	if (__builtin_mul_overflow(threads_count, sizeof(struct thread_info), &threadpool_size) ||
	    __builtin_add_overflow(threadpool_size, sizeof(struct pthreadpool), &threadpool_size)) {
	    return NULL;
	}

or the original reporter's

+	if (threads_count > (UINT_MAX - sizeof(struct pthreadpool)) / sizeof(struct thread_info)) {
+		return NULL;
+	}

(which is more conservative because UINT_MAX isn't the right constant on LP64.)

Start in src/memory.c at the allocation used by pthreadpool_create(), using the linked commit and original bug report for context. Verify the size calculation for the thread pool and confirm that oversized values are rejected by returning NULL without a heap-buffer-overflow.