fix: correct buffer size in archiveSnapshot

[Qian-Cheng-nju]

Hi!

I found a small buffer sizing issue in archiveSnapshot() and put together a fix.

The backup filename buffer is allocated with strlen(rdb_filename) bytes, but snprintf writes "%s.bak.%d" which needs extra space for ".bak." and the node ID. This truncates the filename on every call (e.g. "redisraft.rdb" becomes "redisraft.rd"), so rename() targets the wrong path and snapshot backups are silently lost.

The fix allocates enough space for the suffix.

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