PVS-Studio warning V547 for NetXMS

int ipfix_snprint_string(....)
{
  size_t  i;
  uint8_t *in = (uint8_t*) data;

  for( i=len-1; i>=0; i-- ) {
    if ( in[i] == '\0' ) {
      return snprintf( str, size, "%s", in );
    }
  }
  ....
}

This suspicious code was found in NetXMS project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'i >= 0' is always true. Unsigned type value is always >= 0. ipfix.c 488

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