PVS-Studio warning V547 for DOSBox

void SERIAL_getErrorString(char* buffer, int length) {
  ...
  if((length - sysmsg_offset -
      strlen((const char*)sysmessagebuffer)) >= 0)
     memcpy(buffer + sysmsg_offset, sysmessagebuffer,
            strlen((const char*)sysmessagebuffer));
  ...
}

This is an example of unsafe code from the viewpoint of buffer overflows. The strlen() function returns size_t. It means that the check for buffer overflow doesn't work.

This suspicious code was found in DOSBox project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression is always true. Unsigned type value is always >= 0. dosbox libserial.cpp 155

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/