PVS-Studio warning V595 for ACE

JAWS_IO_Reactive_Transmit::handle_output_source
  (ACE_HANDLE handle)
{
  ACE_Message_Block *mb = this->source_buf_;
  ...
  if (mb->length () > 0)
    result = this->handle_output_mb (handle, mb);
  if (result < 0) {
    ...
  } else if (mb == 0 && this->source_ == ACE_INVALID_HANDLE)
    this->source_buf_ = 0;
  ...
}

This suspicious code was found in ACE project by PVS-Studio static code analyzer.
Warning message is:
V595 The 'mb' pointer was utilized before it was verified against nullptr. Check lines: 455, 463. JAWS3 reactive_io.cpp 455

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