[app] Forget exchange context when SubscribeResponse is received

SubscribeResponse is the last action in the Subscribe
transaction and ExchangeContext::HandleMessage
automatically closes exchanges that have no pending
requests nor responses.

ReadClient still kept the pointer to the exchange after
receiving SubscribeResponse, so it would attempt to abort
already closed exchange if the subscription was cancelled
after receiving the SubscribeResponse and before receving
a subsequent Report message.

src/app/ReadClient.cpp

@@ -300,6 +300,10 @@ CHIP_ERROR ReadClient::OnMessageReceived(Messaging::ExchangeContext * apExchange
     {
         VerifyOrExit(apExchangeContext == mpExchangeCtx, err = CHIP_ERROR_INCORRECT_STATE);
         err = ProcessSubscribeResponse(std::move(aPayload));
+        // Forget the context as SUBSCRIBE RESPONSE is the last message in SUBSCRIBE transaction and
+        // ExchangeContext::HandleMessage automatically closes a context if other messages need to
+        // be sent or received.
+        mpExchangeCtx = nullptr;
         SuccessOrExit(err);
     }
     else