qsort5.c pthread_create (etc) return values not checked

[andrew-appel]

In the qsort5.c benchmark, the return values from pthread_create, pthread_mutex_lock, etc. are not checked; so if those functions experience errors, the program fails.

Also, because pthread_join is after the decrement of active, the number of threads could be greater than THREADS (although, technically, the thread doing pthread_join is blocked, not active...)