cuserid: support invocation with a null pointer argument

[musl] / src / legacy / cuserid.c

#define _GNU_SOURCE
#include <pwd.h>
#include <stdio.h>
#include <unistd.h>

char *cuserid(char *buf)
{
        static char usridbuf[L_cuserid];
        struct passwd pw, *ppw;
        long pwb[256];
        if (getpwuid_r(geteuid(), &pw, (void *)pwb, sizeof pwb, &ppw))
                return 0;
        if (!buf) buf = usridbuf;
        snprintf(buf, L_cuserid, "%s", pw.pw_name);
        return buf;
}