PVS-Studio warning V595 for LAME 3.xx

static int
check_vbr_header(PMPSTR mp,int bytes)
{
  ...
  buf  = buf->next;
  pos = buf->pos;
  if(!buf) return -1; /* fatal error */
  ...
}

This suspicious code was found in LAME 3.xx project by PVS-Studio static code analyzer.
Warning message is:
V595 The 'buf' pointer was utilized before it was verified against nullptr. Check lines: 226, 227.  mpglib interface.c 226

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