Don't copy null terminators from the log's `header.gameid` into `FifoDataFile`'s `m_game_id`. Doing so would cause an infinite loop in `Core::GenerateScreenshotName` as the various concatenations and `fmt::format` calls would then effectively drop all the timestamps and disambiguating arbitrary numbers since they followed the null terminator in the gameid, and so the `File::Exists` calls would always return true. Fixes https://bugs.dolphin-emu.org/issues/14002.