Fix a UB in BufferedWriter::Write

[vitaut]

Fix a UB in BufferedWriter::Write:

    #0 0x44c318 in BufferedWriter::Write(void const*, unsigned long) .../BuildEvents.cpp:473
    #1 0x44b615 in SaveBuildEvents(BuildEventsParser*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) .../BuildEvents.cpp:556
    #2 0x59b0ca in ProcessJsonFiles(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, long, long) .../main.cpp:194
    ...

which is caused by passing a null pointer to memcpy.