valgrind realloc错误:条件跳转或移动取决于未初始化的值

==17209== Conditional jump or move depends on uninitialised value(s)
==17209==    at 0x402E7C5: __GI___rawmemchr (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==17209==    by 0x40CE921: _IO_str_init_static_internal (strops.c:45)
==17209==    by 0x40B0B76: __isoc99_vsscanf (isoc99_vsscanf.c:42)
==17209==    by 0x8048647: main (lala.c:23)
==17209==  Uninitialised value was created by a stack allocation
==17209==    at 0x8048659: gatherInfoSalt (lala.c:28)
==17209== 

.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include <string.h>
#include <unistd.h>
#include <errno.h>


struct sysinfo_s {
    char *salt_id;
};


void gatherInfoSalt(char ** );

int main (int argc, char *argv[]) {
    struct sysinfo_s si;

    si.salt_id = malloc(1);
    *si.salt_id = 0;

    gatherInfoSalt(&si.salt_id);

    free(si.salt_id);
}

void gatherInfoSalt(char ** salt_id) {
    FILE * fp;
    char buf[256];
    int r;


    if (!(fp = fopen("/etc/salt/minion.d/id.conf","r"))) {
            perror("fopen");
        return;
    }

    r=fread(buf,1,255,fp);

    //adding *buf=0 here removes the error (but removes functionality)

    if (r) {
        *salt_id=realloc(*salt_id,r-2);
        **salt_id=0;
        sscanf(buf,"id:%s",*salt_id);
        //sscanf(buf,"id:%s",buf1);
    }

    printf("saltid: %s\n",*salt_id);

    fclose(fp);

}