redis_3.0.7_sds.c_sdsclear()

//将sds字符串置空    
void sdsclear(sds s)    
{    
    struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));    
   sh->free += sh->len;        
    sh->len = 0;    
   sh->buf[0]='\0';    
}