Fix the versioned save reader for an RPG

Your RPG wrote saves field by field with no version and no item count. When a patch added one field, every old save loaded with a broken inventory. The new format starts with a version number and counts the inventory, so the reader knows where each part ends.

An assistant wrote read_save_record(record). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• record: a save, as comma-separated integers: version, level, health, item count, that many item ids, gold, and for version 2 only, mana.
• Return ["level=<n>", "health=<n>", "inventory=<ids joined by ;>", "gold=<n>", "mana=<n>"], or a single error string.

Rules the tests check, in this order:

1. An empty record is ["error=corrupt"]. Split the rest on ,. Fewer than 5 parts is ["error=corrupt"].
2. Every part must be an integer: an optional -, then one or more ASCII digits. Otherwise ["error=corrupt"].
3. A version below 1 is ["error=corrupt"]. A version above 2 is ["error=unsupported_version"].
4. A negative item count is ["error=corrupt"].
5. The record must have exactly 5 parts plus the item count for version 1, or exactly 6 parts plus the item count for version 2. Any other length is ["error=corrupt"].

• Write every number in the output in plain form, as the integer it parses to: 007 becomes 7 and -0 becomes 0.
• A version 1 save has no mana field, so mana is 0.