Fix: billing shop boulder pushed thru shared wall

I realized that the "move the boulder billing to 'used-up items'" part
of 20392a6 didn't properly handle boulder movement from one shop to
another via a gap in a shared wall, becuase it wasn't looking at the
complete in_rooms() array (plus, it only triggered if the new spot
wasn't in any shops at all).  When I tried to fix that, I realized that
stolen_value() was similarly not working reliably when passed a location
shared between two shops, and for the same reason: it was only using the
first character of the in_rooms() array.

I think this patch fixes both those things, but it would be worth
examining the change to stolen_value() carefully, to ensure getting the
roomno via the shkp won't change anything about its normal functioning.
I'm not totally sure about that -- I didn't notice any problems in some
brief tests of typical stolen_value() uses, but it seems like it
probably has some tricky edge cases.

At the very least, passing a boulder fully through a shared wall between
two shops one way, then back the other way, no longer triggers an unpaid
obj sanity check in my testing.

src/hack.c

@@ -462,8 +462,10 @@ moverock(void)
                            != 0)
                        && onshopbill(otmp, shkp, TRUE)) {
                 subfrombill(otmp, shkp);
-            } else if (otmp->unpaid && !*in_rooms(rx, ry, SHOPBASE)
-                       && *in_rooms(sx, sy, SHOPBASE)) {
+            } else if (otmp->unpaid
+                       && (shkp = find_objowner(otmp, sx, sy)) != 0
+                       && !strchr(in_rooms(rx, ry, SHOPBASE),
+                                  ESHK(shkp)->shoproom)) {
                 /* once the boulder is fully out of the shop, so that it's
                  * impossible to change your mind and push it back in without
                  * leaving and triggering Kops, switch it to stolen_value */