github issue #1060 - crystal helmet

Issue reported by vultur-cadens:  changing helm of brilliance to
crystal made it stop being classified as "hard helmet" so it gave
less protection against things falling onto the hero's head.

Change the is_metallic() tests used on helmets to new hard_helmet().
Unlike when thrown, crystal helmets don't break when objects fall
on them.

Fixes #1060

src/zap.c

@@ -3148,7 +3148,7 @@ zap_updown(struct obj *obj) /* wand or spell */
             /* similar to zap_dig() */
             pline("A rock is dislodged from the %s and falls on your %s.",
                   ceiling(x, y), body_part(HEAD));
-            dmg = rnd((uarmh && is_metallic(uarmh)) ? 2 : 6);
+            dmg = rnd(hard_helmet(uarmh) ? 2 : 6);
             losehp(Maybe_Half_Phys(dmg), "falling rock", KILLED_BY_AN);
             if ((otmp = mksobj_at(ROCK, x, y, FALSE, FALSE)) != 0) {
                 (void) xname(otmp); /* set dknown, maybe bknown */