nullcomp's passthrough path returns non-cmp-header data as-is without
error, which is correct for old uncompressed saves. However, a corrupt
backup slot containing garbage shorter than the minimum save layout
(100 bytes) would pass Decompress() and then panic in
updateStructWithSaveData() with a slice-bounds error at the name field
read (offset 88–100).
Add a minSaveSize check after each backup decompresses; skip the slot
if the result is too small. Also document the campaign system and the
fix in CHANGELOG under Unreleased.
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