DS::FileStream::atEof() UB

[Hoikas]

DS::FileStream::atEof() relies on undefined behavior if the file is opened in read mode. It basically tests for EOF by trying to read a character from the file and put the character back for future read attempts. In Ubuntu 20.04 on WSL2, this causes the character read by atEof() to be discarded. This is also true for Alpine running in a docker container on WSL2.