boulders vs vault walls

Noticed when testing the "wall_angle: unknown" fix, if there is a
boulder rather than a door in the breach in a vault's walls at the
spot where the guard arrives, the guard would walk onto it, tell the
hero to drop any gold and follow, then move back.  The boulder would
remain in the hero's way and couldn't be pushed because the guard was
in the boulder's way.  Have the guard smash any such boulder(s) into
rocks when arriving (with no explanation for how that is accomplished,
just a message about seeing or hearing boulder(s) shatter).  Later
when repairing the walls, delete any rocks or boulders at all vault
wall locations (even when no gap-to-wall repair is needed).

src/zap.c

@@ -4466,14 +4466,7 @@ melt_ice(xchar x, xchar y, const char *msg)
     if (lev->typ == DRAWBRIDGE_UP || lev->typ == DRAWBRIDGE_DOWN) {
         lev->drawbridgemask &= ~DB_ICE; /* revert to DB_MOAT */
     } else { /* lev->typ == ICE */
-#ifdef STUPID
-        if (lev->icedpool == ICED_POOL)
-            lev->typ = POOL;
-        else
-            lev->typ = MOAT;
-#else
         lev->typ = (lev->icedpool == ICED_POOL ? POOL : MOAT);
-#endif
         lev->icedpool = 0;
     }
     spot_stop_timers(x, y, MELT_ICE_AWAY); /* no more ice to melt away */
@@ -4877,7 +4870,7 @@ zap_over_floor(xchar x, xchar y, int type, boolean *shopdamage,
     return rangemod;
 }
 
-/* fractured by pick-axe or wand of striking */
+/* fractured by pick-axe or wand of striking or by vault guard */
 void
 fracture_rock(struct obj *obj) /* no texts here! */
 {