spell_damage_bonus (trunk only)

From the newsgroup:  player saw "The spell hits the <monster>?"
where the question mark punctuation reflected negative damage occurring.
Another player diagnosed it as a 2 point force bolt (from 2d12 dice role)
modified by -3 penalty for hero who has Int less than 10.  This changes
spell_damage_bonus() to avoid reducing damage below 1 point.

doc/fixes35.0

@@ -320,6 +320,7 @@ prevent temple priests and minions from wearing helms of opposite alignment
 pearl rings shouldn't rust
 shouldn't be able to read a worn T-shirt when it's covered by a worn suit
 simplify hero placement on Castle level when climbing up stairs from Valley
+spell attack by low-Int hero could inflict negative damage
 
 
 Platform- and/or Interface-Specific Fixes