PVS-Studio warning V642 for xpdfwin

GBool BitmapOutputDev::clip0and1differ(....)
{
  ....
  char differs2 = memcmp(a0, a1, width*height);
  if(differs && !differs2)
    msg("<warning> Strange internal error (2)");
  ....
}

This suspicious code was found in xpdfwin project by PVS-Studio static code analyzer.
Warning message is:
V642 Saving the 'memcmp' function result inside the 'char' type variable is inappropriate. The significant bits could be lost breaking the program's logic. bitmapoutputdev.cc 710

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/