improper order to check NULL pointer in libthr

[Jin Guojun]

Most pthread_mutex functions and some pthread_ functions in linbthr do 
not check NULL
pointer at the function entry, but they dereference the pointer to see 
if its content is NULL.
The NULL pointer is ASSERTed at last stage (in xxxxx_common routines) 
somehow, which is
meaningless.

So, what is the correct design pattern? -- (should be like libpthread, 
which looks correct)

Return EFAULT / EINVAL at the entry point as +++ below if the pointer is 
NULL?
or
ASSERT at the entry point if pointer or its content is NULL?

Please CC me since I am not on thread list.

Thanks,
-Jin

int
_pthread_mutex_trylock(pthread_mutex_t *mutex)
{
+       if (!mutex)
+              return  EFAULT;
+
        struct pthread  *curthread = _get_curthread();
        int     ret = 0;

        /*
         * If the mutex is statically initialized, perform the dynamic
         * initialization marking the mutex private (delete safe):
         */
        if ((*mutex != NULL) ||
            ((ret = init_static_private(curthread, mutex)) == 0))
                ret = mutex_trylock_common(curthread, mutex);
         
        return (ret);
}

static int
mutex_lock_common(struct pthread *curthread, pthread_mutex_t *m,
        const struct timespec * abstime)
{
        struct  timespec ts, ts2;
        long    cycle;
        int     ret = 0;

        THR_ASSERT((m != NULL) && (*m != NULL),
            "Uninitialized mutex in mutex_lock_common");