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 */