Your assumption is incorrect to start with. pthread_t objects are opaque. You cannot compare pthread_t types directly in C. You should use pthread_equal instead.
Another consideration is that if pthread_create fails, the contents of your pthread_t will be undefined. It may not be set to your invalid value any more.
My preference is to keep the return values of the pthread_create calls (along with the thread IDs) and use that to determine whether each thread was started correctly.