PVS-Studio warning V547 for PHP

static size_t php_ftp_dirstream_read(....)
{
  size_t tmp_len;
  ....
  /* Trim off trailing whitespace characters */
  tmp_len--;
  while (tmp_len >= 0 &&                  //<==
    (ent->d_name[tmp_len] == '\n' ||
     ent->d_name[tmp_len] == '\r' ||
     ent->d_name[tmp_len] == '\t' ||
     ent->d_name[tmp_len] == ' ')) {
       ent->d_name[tmp_len--] = '\0';
  }
  ....
}

This suspicious code was found in PHP project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'tmp_len >= 0' is always true. Unsigned type value is always >= 0. ftp_fopen_wrapper.c 639

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