PVS-Studio warning V547 for MongoDB

void* MemoryMappedFile::map(....) {
  ....
  size_t len = strlen( filename );
  for ( size_t i=len-1; i>=0; i-- ) {
    if ( filename[i] == '/' ||
         filename[i] == '\\' )
      break;

    if ( filename[i] == ':' )
         filename[i] = '_';
  }
  ....
}

This suspicious code was found in MongoDB project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'i >= 0' is always true. Unsigned type value is always >= 0. mmap_win.cpp 90

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/