cut out a syscall on thread creation in the case where guard size is 0

[musl] / src / thread / pthread_mutex_lock.c

#include "pthread_impl.h"

int pthread_mutex_lock(pthread_mutex_t *m)
{
        int r;
        while ((r=pthread_mutex_trylock(m)) == EBUSY)
                __wait(&m->_m_lock, &m->_m_waiters, 1, 0);
        return r;
}