Fix spell absorption capping

[petchema]

When absorbing magicka from own spells, DFU explicitly ensures that magicka recovered is not larger than the magicka that was used. For this, it needs to track how much magicka was spent for the last spell: lastReadySpellCastingCost

A special case was not correctly handled though: when the last spell came from a magic item, lastReadySpellCastingCost should be set 0 so that next absorption of own spells is not capped.
That worked by luck when the game was just started because lastReadySpellCastingCost was still 0, but broke after first spell casting.

Forums: https://forums.dfworkshop.net/viewtopic.php?t=6755

[petchema]

Test protocol:

• notice current spellpoints
• use the magic item to cast fireball at the facing wall
• check that spellpoints increased by 62, cost of the fireball spell for the character
• cast slowfall from the spellbook
• notice current spellpoints
• use the magic item to cast fireball at the facing wall
• check that spellpoints increased by less than 62 (actually increased by the cost of slowfall)

(sorry for the traces of french translation mod in the save)
SAVE37.zip