Truncate existing temp file in meta_partition constructor

[adamant-pwn]

Ensure the temp file is truncated if it exists. Otherwise, repeated independent runs would add lines to the same file, and it isn't detected when the files are read later on, leading to "seed didn't work" exceptions.

[adamant-pwn]

Might make sense to also add a cleanup to whatever place in the code where it's no longer needed?

UPD: On further inspection, I see that you do clean up with std::remove. Nevertheless, I ran into some issues locally when the files somehow still existed (perhaps due to unexpected termination in previous runs), and in such cases if it runs into failure again (typically the "seed did not work" situation), it won't clean them up again due to consequent early terminations, so making sure they're empty in the constructor would still make sense.

[jermp]

Ok, makes sense, thanks :)