spell.max conversion from int16 to int32

Converting the spell.max field (spdat.h) to int32 to handle larger numbers in the
PercentalHeal case.

The PercentalHeal case in spell_effects.cpp has "int32 val = spell.max[i]". However, the 'spell.max' field was int16, which in large number cases, resulted in highly unpredictable results. 

I couldn't see any immediate problems as a result of this change.

-Hate