spells doing negative damage

Add a check to zhitm to ensure that spells cast by characters with low XL
and Int will not do negative damage.

doc/fixes33.2

@@ -399,6 +399,7 @@ monsters shouldn't use wands of digging in Sokoban
 objects dropped in or travelling across java pools can take damage
 monsters that enter lava can take damage
 eating an unpaid tin should calculate cost before not after eating
+spells shouldn't do negative damage
 
 
 Platform- and/or Interface-Specific Fixes

src/zap.c

@@ -2981,6 +2981,7 @@ struct obj **ootmp;	/* to return worn armor for caller to disintegrate */
 	if (tmp > 0 && type >= 0 &&
 		resist(mon, type < ZT_SPELL(0) ? WAND_CLASS : '\0', 0, NOTELL))
 	    tmp /= 2;
+	if (tmp < 0) tmp = 0;		/* don't allow negative damage */
 #ifdef WIZ_PATCH_DEBUG
 	pline("zapped monster hp = %d (= %d - %d)", mon->mhp-tmp,mon->mhp,tmp);
 #endif