more shop billing object sanity

Used up items moved to the billobjs list still have obj->unpaid set.
That should probably be cleared since it has no meaning there, but
this hasn't done that.

For those keeping score:  unpaid checking has triggered three false
positives (so far) and found one bug.

src/mkobj.c

@@ -2800,6 +2800,8 @@ shop_obj_sanity(struct obj *obj, const char *mesg)
     /* these will always be needed for the normal case, so don't bother
        waiting until we find an insanity to fetch them */
     shkp = find_objowner(obj, x, y);
+    if (shkp && obj->where == OBJ_ONBILL)
+        x = shkp->mx, y = shkp->my;
     costly = costly_spot(x, y);
     costlytoo = costly_adjacent(shkp, x, y);
 
@@ -2808,11 +2810,13 @@ shop_obj_sanity(struct obj *obj, const char *mesg)
         why = "%s obj both unpaid and no_charge! %s %s: %s";
     } else if (obj->unpaid) {
         /* unpaid is only applicable for directly carried objects, for
-           objects inside carried containers, and for floor items outside
+           objects inside carried containers, for used up items on the
-           the shop proper but within the shop boundary (walls, door, "free
+           billobjs list, and for floor items outside the shop proper
-           spot") and for objects moved from such spots into the shop proper
+           but within the shop boundary (walls, door, "free spot") and
-           by repair of shop walls */
+           for objects moved from such spots into the shop proper by
+           repair of shop walls */
         if (otop->where != OBJ_INVENT
+            && obj->where != OBJ_ONBILL /* when on bill, obj==otop */
             && (otop->where != OBJ_FLOOR || (!costly && !costlytoo)))
             why = "%s unpaid obj not carried! %s %s: %s";
         else if (!costly && !costlytoo)

src/shk.c

@@ -872,19 +872,28 @@ find_objowner(
                            * they shouldn't be modified */
 {
     struct monst *shkp, *deflt_shkp = 0;
-    char *roomindx, *where = in_rooms(x, y, SHOPBASE);
 
-    /* conceptually object could be inside up to four rooms simultaneously;
+    if (obj->where == OBJ_ONBILL) {
-       in practice it will usually be one room but can sometimes be two;
+        /* used up item; bill obj coordinates are useless and so are x,y */
-       check shk and bill for each room rather than just the first;
+        for (shkp = next_shkp(fmon, TRUE); shkp;
-       fallback to the first shk if obj isn't on the relevant bill(s) */
+             shkp = next_shkp(shkp->nmon, TRUE))
-    for (roomindx = where; *roomindx; ++roomindx)
-        if ((shkp = shop_keeper(*roomindx)) != 0) {
             if (onshopbill(obj, shkp, TRUE))
                 return shkp;
-            if (!deflt_shkp)
+    } else {
-                deflt_shkp = shkp;
+        char *roomindx, *where = in_rooms(x, y, SHOPBASE);
-        }
+
+        /* conceptually object could be inside up to 4 rooms simultaneously;
+           in practice it will usually be one room but can sometimes be two;
+           check shk and bill for each room rather than just the first;
+           fallback to the first shk if obj isn't on the relevant bill(s) */
+        for (roomindx = where; *roomindx; ++roomindx)
+            if ((shkp = shop_keeper(*roomindx)) != 0) {
+                if (onshopbill(obj, shkp, TRUE))
+                    return shkp;
+                if (!deflt_shkp)
+                    deflt_shkp = shkp;
+            }
+    }
     return deflt_shkp;
 }