sanity checking: obfree() vs obj->in_use

Drinking a potion sets the in_use flag so that it can be deleted
if found in a hangup save file.  That flag wasn't being cleared if
a shop-owned item is used up and goes onto the shop bill instead of
being discarded.  The recently revised object sanity checking was
complaining (turn after turn after turn until player paid for the
potion) that there was an object with its in_use bit set.

Restore only purges in_use objects in invent and didn't mess up
billing by getting rid such objects since they're on another list.

src/shk.c

@@ -1,4 +1,4 @@
-/* NetHack 3.7	shk.c	$NHDT-Date: 1629496872 2021/08/20 22:01:12 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.204 $ */
+/* NetHack 3.7	shk.c	$NHDT-Date: 1629548922 2021/08/21 12:28:42 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.205 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Robert Patrick Rankin, 2012. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -2447,7 +2447,7 @@ unpaid_cost(
 }
 
 static void
-add_one_tobill(struct obj* obj, boolean dummy, struct monst* shkp)
+add_one_tobill(struct obj *obj, boolean dummy, struct monst *shkp)
 {
     struct eshk *eshkp;
     struct bill_x *bp;
@@ -2459,6 +2459,11 @@ add_one_tobill(struct obj* obj, boolean dummy, struct monst* shkp)
 
     if (eshkp->billct == BILLSZ) {
         You("got that for free!");
+        /*
+         * FIXME:
+         *  What happens when this is a dummy object?  It won't be on any
+         *  object list.
+         */
         return;
     }
 
@@ -2496,6 +2501,11 @@ add_to_billobjs(struct obj* obj)
     obj->nobj = g.billobjs;
     g.billobjs = obj;
     obj->where = OBJ_ONBILL;
+
+    /* if hero drinks a shop-owned potion, it will have been flagged
+       in_use by dodrink/dopotion but isn't being be used up yet because
+       it stays on the bill; only object sanity checking actually cares */
+    obj->in_use = 0;
 }
 
 /* recursive billing of objects within containers. */