PVS-Studio warning V595 for LAME 3.xx

static int read_buf_byte(PMPSTR mp)
{
  ...
  pos = mp->tail->pos;
  while(pos >= mp->tail->size) {
    remove_buf(mp);
    if(!mp->tail) {
      fprintf(stderr,
        "Fatal error! tried to read past mp buffer\n");
      exit(1);
    }
    pos = mp->tail->pos;
  }
  ...
}

This suspicious code was found in LAME 3.xx project by PVS-Studio static code analyzer.
Warning message is:
V595 The 'mp->tail' pointer was utilized before it was verified against nullptr. Check lines: 136, 139.  mpglib interface.c 136

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/