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.