fix #H3039 - panic() when trying to drop destroyed items

From a bug report, dropping a lit
(burning) potion of oil while levitating can produce an explosion which can
destroy inventory.  If in the process of dropping multiple items, the ones
after the oil might be gone, resulting in use of stale pointers and possibly
triggering an "extract_nobj: object lost" panic or even a crash.  While
testing my fix, I discovered that being killed by an exploding potion of oil
could produce an "object_is_local" panic if bones are saved  (and reproduced
with unmodified 3.4.3).

src/zap.c

@@ -1710,6 +1710,9 @@ struct obj *obj, *otmp;
 		 *	       meat; prevent those contents from being hit.
 		 * retouch_equipment() - bypass flag is used to track which
 		 *	       items have been handled (bhito isn't involved).
+		 * menu_drop(), askchain() - inventory traversal where multiple
+		 *	       Drop can alter the invent chain while traversal
+		 *	       is in progress (bhito isn't involved).
 		 *
 		 * The bypass bit on all objects is reset each turn, whenever
 		 * context.bypasses is set.