And it should. You're referencing an unitialized pointer, the behavior of which is undefined. You're lucky your program didn't send an e-mail bomb to your employer about the use of company facilities.
Try this instead:
FILE *f = popen("xmmsctrl title", "r");
// Note: should really check if f == NULL here.
fgets(buf, sizeof(buf), f)
printf("Now playing: %s", buf);