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/worn.c

@@ -673,6 +673,35 @@ struct obj *obj;
 	context.bypasses = TRUE;
 }
 
+/* set or clear the bypass bit in a list of objects */
+void
+bypass_objlist(objchain, on)
+struct obj *objchain;
+boolean on;	/* TRUE => set, FALSE => clear */
+{
+	if (on && objchain) context.bypasses = TRUE;
+	while (objchain) {
+	    objchain->bypass = on ? 1 : 0;
+	    objchain = objchain->nobj;
+	}
+}
+
+/* return the first object without its bypass bit set; set that bit
+   before returning so that successive calls will find further objects */
+struct obj *
+nxt_unbypassed_obj(objchain)
+struct obj *objchain;
+{
+	while (objchain) {
+	    if (!objchain->bypass) {
+		bypass_obj(objchain);
+		break;
+	    }
+	    objchain = objchain->nobj;
+	}
+	return objchain;
+}
+
 void
 mon_break_armor(mon, polyspot)
 struct monst *mon;