Avoid bad access deref in swapchest 'A'

If a player elected to remove multiple items from the swapchest, whether
by "A" or by selecting more than one in the menu, and the _second_ one
in the list would have put her over the weight limit (i.e. would prompt
"Continue?"), selecting 'n' rather than 'y' or 'q' to that prompt could
crash the game as it tried to continue down the chain of swap items
after they had been freed.  Add some further tests to avoid this: do the
"swap chest is dormant" check first, before testing the weight and
prompting, and return -1 to stop attempting to continue down the cobj
linked list when an item is removed from the swapchest.

src/pickup.c

@@ -2669,6 +2669,16 @@ register struct obj *obj;
         obj->owt = weight(obj);
     }
 
+    if (current_container->otyp == SWAP_CHEST
+        && current_container->swapitems < SWAP_ITEMS_MIN) {
+        /* should not get to here if we haven't contributed to the chest */
+        You_feel(current_container->swapitems == SWAP_CHEST_USED_UP
+                    ? "%s is no longer interested in dealing with you."
+                    : "%s wants something from you first.",
+                    the(xname(current_container)));
+        return -1;
+    }
+
     if (obj->oartifact && !touch_artifact(obj, &youmonst))
         return 0;
 
@@ -2685,14 +2695,6 @@ register struct obj *obj;
 
     /* TNNT swap chest --> */
     if (current_container->otyp == SWAP_CHEST) {
-        if (current_container->swapitems < SWAP_ITEMS_MIN) {
-            /* should not get to here if we haven't contributed to the chest */
-            You_feel(current_container->swapitems == SWAP_CHEST_USED_UP
-                       ? "%s is no longer interested in dealing with you."
-                       : "%s wants something from you first.",
-                     the(xname(current_container)));
-            return -1;
-        }
         if (!delete_swapobj_file(obj)) {
             /* fails if file already doesn't exist */
             pline("You reach for %s, but %s!",
@@ -2778,6 +2780,7 @@ register struct obj *obj;
                        prefix, itemname);
         if (save_oname && !obj->oartifact)
             ONAME(obj) = save_oname;
+        return -1; /* don't try to remove any more items */
     }
     /* <-- */