vtkOpenFOAMReader: fix 32-bit compilation on Windows

#af7913ab introduced the following error when building for 32-bit with MSVC

  error C2666: 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator []': 3 overloads have similar conversions
  note: could be 'const char &std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator [](const unsigned int) const'
  note: or       'char &std::basic_string<char,std::char_traits<char>,std::allocator<char>>::operator [](const unsigned int)'
  note: or       'built-in C++ operator[(const char *, int)'
  note: while trying to match the argument list '(vtkStdString, int)'

Whether 0 in "[0]" corresponds to "int" or "unsigned int" seems ambiguous.
Reverting the change to avoid using operator [].