stone-to-flesh vs Heart of Ahriman (trunk only)

From the newsgroup:  a player accidentally cast the stone-to-flesh
spell at himself (I don't recall whether he chose wrong spell or wrong
direction, or tried to cancel and game used last remembered direction)
and the barbarian quest artifact he was carring turned into a meatball.
Artifacts already have a high chance (95%) to resist being polymorphed
but that doesn't apply for the stone-to-flesh transformation.  This
gives stone artifacts a high chance (98%) to resist being turned into
flesh.  Non-artifacts also get a small chance (2%) to resist as well.

src/zap.c

@@ -1565,6 +1565,8 @@ struct obj *obj;
 
     if (objects[obj->otyp].oc_material != MINERAL &&
 	    objects[obj->otyp].oc_material != GEMSTONE) return 0;
+    /* Heart of Ahriman usually resists; ordinary items rarely do */
+    if (obj_resists(obj, 2, 98)) return 0;
 
     (void) get_obj_location(obj, &oox, &ooy, 0);
     /* add more if stone objects are added.. */