allow locks to be forced again

- use uwep's oc_wldam value, not chest's oc_wldam value, for computing chance.
  oc_wldam has always been 0 for all containers
- the code was broken for a long time, but was masked by the off-by-1 error
  in checking the chance, allowing a 1% chance of forcing
- the result is that forcing locks is a lot easier

src/lock.c

@@ -476,7 +476,7 @@ doforce()		/* try to force a chest with your weapon */
 		else
 		    You("start bashing it with your %s.", xname(uwep));
 		xlock.box = otmp;
-		xlock.chance = objects[otmp->otyp].oc_wldam * 2;
+		xlock.chance = objects[uwep->otyp].oc_wldam * 2;
 		xlock.picktyp = picktyp;
 		xlock.usedtime = 0;
 		break;