PVS-Studio warning V547 for Virtualdub

typedef unsigned short wint_t;

void lexungetc(wint_t c) {
  if (c < 0)
    return;

  g_backstack.push_back(c);
}

This suspicious code was found in Virtualdub project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'c < 0' is always false. Unsigned type value is never < 0. Ami lexer.cpp 225

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/