Clear tin-eating struct when object goes away

The tin-eating context was pointing to a non-existent object,
causing an error when the fuzzer somehow managed to continue eating
the freed tin object.
Clear the pointer when the tin leaves inventory or the object
is deleted.

src/invent.c

@@ -1373,6 +1373,11 @@ freeinv_core(struct obj *obj)
     } else if (obj->otyp == FIGURINE && obj->timed) {
         (void) stop_timer(FIG_TRANSFORM, obj_to_any(obj));
     }
+
+    if (obj == svc.context.tin.tin) {
+        svc.context.tin.tin = (struct obj *) 0;
+        svc.context.tin.o_id = 0;
+    }
 }
 
 /* remove an object from the hero's inventory */

src/mkobj.c

@@ -2711,6 +2711,10 @@ dealloc_obj(struct obj *obj)
         gt.thrownobj = 0;
     if (obj == gk.kickedobj)
         gk.kickedobj = 0;
+    if (obj == svc.context.tin.tin) {
+        svc.context.tin.tin = (struct obj *) 0;
+        svc.context.tin.o_id = 0;
+    }
 
     /* if obj came from the most recent splitobj(), it's no longer eligible
        for unsplitobj(); perform inline clear_splitobjs() */