stone to flesh bit

Since the crysknife is the only MINERAL object in the
game that isn't affected by the "stone to flesh" spell,
and the only thing that makes it to the default
case on the switch statement in zap.c, make it
obvious that it isn't an oversight that nothing happens.
(it wasn't an oversight, right?)

src/zap.c

@@ -1674,6 +1674,8 @@ smell:
 			else
 			    Norep("You smell a delicious smell.");
 			break;
+		    case WEAPON_CLASS:	/* crysknife */
+		    	/* fall through */
 		    default:
 			res = 0;
 			break;