fix issue #1339 - shop purchase warnings

Issue reported by ars3niy:  with the relatively new container
handling, buying multiple items when some were inside a container
sometimes triggered impossible "unpaid_cost: object wasn't on any
bill" warnings and not buy all intended items.  Once that occurred,
subsequent inventory display would repeat the warning.

A couple weeks back, I managed to produce a save file which would
reproduce the problem when 'p' was issued, but failed to figure
out why that was happening.  I accidentally deleted that save file
and it took quite a lot of further attempts to get another one.

I still don't understand why this fix is needed, but with it in
place the save file no longer triggers any problems.  I'm marking
the issue fixed but that could be premature.

Fixes #1339

doc/fixes3-7-0.txt

@@ -2081,6 +2081,8 @@ farlook of /e or /E stopped reporting engraving and headstone text after a fix
 	for the situation where // or ; reported such text when there was an
 	object or monster in the way
 message was garbled when lightning or acid breath hit iron bars
+shop bug: buying a container with unpaid items in it could produce impossible
+	"unpaid_cost: object not on any bill" warnings
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository

src/shk.c

@@ -27,7 +27,7 @@ enum billitem_status {
     KnownContainer = 5, /* container->cknown==1, holding unpaid item(s) */
     UndisclosedContainer = 6, /* container->cknown==0 */
 };
-/* this is similar to sortloot; the shop bill gets converted into a array of
+/* this is similar to sortloot; the shop bill gets converted into an array of
    struct sortbill_item so that sorting and traversal don't need to access
    the original bill or even the shk; the array gets sorted by usedup vs
    unpaid and by cost within each of those two categories */
@@ -1570,7 +1570,8 @@ make_itemized_bill(
     }
     ibill[n].bidx = -1; /* end of list; not strictly needed */
 
-    /* ibill[0..n-1] contains data, ibill[n] has Null obj and -1 bidx */
+    /* ibill[0..n-1] contains data, ibill[n] has Null obj and -1 bidx and
+       is excluded from the sort */
     if (n > 1)
         qsort((genericptr_t) ibill, n, sizeof *ibill, sortbill_cmp);
     return n;
@@ -2116,9 +2117,13 @@ update_bill(
         }
         newebillct = eshkp->billct - 1;
         *bp = eshkp->bill_p[newebillct];
+#if 0   /* [this is responsible for github issue #1339; it's not clear why] */
         for (j = 0; j < ibillct; ++j)
             if (ibill[j].bidx == newebillct)
                 ibill[j].bidx = bidx;
+#else
+        nhUse(bidx);
+#endif
         eshkp->billct = newebillct; /* eshkp->billct - 1 */
     }
     return;
@@ -2294,9 +2299,13 @@ buy_container(
         otmp = bp_to_obj(bp);
 
         buy = dopayobj(shkp, bp, otmp, 1, FALSE, sightunseen);
-        if (buy != PAY_BUY)
+        if (buy != PAY_BUY) {
             impossible("Buying %s contents failed unexpectedly (#%u %d).",
                        simpleonames(container), otmp->o_id, buy);
+            continue;
+        }
+        /* [updating cost here is not necessary but useful when debugging] */
+        ibill[indx].cost -= (bp->price * bp->bquan); /* update container */
         update_bill(indx, ibillct, ibill, eshkp, bp, otmp);
         ++buycount;
     }