PVS-Studio warning V547 for Firefox

int
PatchFile::LoadSourceFile(FILE* ofile)
{
  ...
  size_t c = fread(rb, 1, r, ofile);
  if (c < 0) {
    LOG(("LoadSourceFile: error reading destination file: "
         LOG_S "\n", mFile));
    return READ_ERROR;
  }
  ...
}

This suspicious code was found in Firefox project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'c < 0' is always false. Unsigned type value is never < 0. updater.cpp 1179

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/