Erroneous zero wipe on first piece of torrent member

[YakoYakoYokuYoku]

Please provide the following information

libtorrent version (or branch): RC_2_0

platform/architecture: Linux 6.12.9

compiler and compiler version: Clang 19.1.7

Might be related to #7843. Another issue I've came across while testing my GStreamer BitTorrent streaming plugin is that when I did a full download of a prioritized file of a torrent I've caught up with the fact that the starting bytes of the file were wiped to zero. I was expecting a Matroska header, but when I hexdumped the file I've only found that the bytes which corresponded to its first piece were all zeroes. That took me by surprise. Such piece was the last of a previous file which was prioritized and downloaded in a previous session. I know that libtorrent uses sparse files when they are incomplete but what I don't is why get zeroed bytes at the beginning of a file that is supposedly complete.

This happened to me once but I don't know how it did. No clue on how to detect this automatically but if there's a way then a solution from the aforementioned issue can be applied in conjunction.