Set xioHandle to NULL when disconnected

[villepalo]

mqtt_client->xioHandle is set in mqtt_client_connect (connection state doesn't matter), so I think it is reasonable to assume it will be reset in mqtt_client_disconnect even if the client isn't connected.

Currently this is causing a segfault in iot-sdk-c library (Azure/azure-iot-sdk-c#446). One could argue, that mqtt_client is not used correctly there, but still I think xioHandle should set to NULL event if clientConnected is false.

[pboettch]

I strongly feel that a unit-test which triggers the problem should be added with such a fix. I had a look at the mqtt_client_ut-part, but I was unable to figure out how to write such a test.

[villepalo]

Test case was a good idea. I realized that there was a requirement for this:
"If the client is disconnected, mqtt_client_dowork shall do nothing."

[pboettch]

And this test-case triggers the problem if xioHandle is not set to NULL?

[villepalo]

This test case reveals that xio_dowork gets called from mqtt_client_dowork, if xioHandle is not set to null, and thus do not meet the requirement ("If the client is disconnected, mqtt_client_dowork shall do nothing").

[jebrando]

I shall pull in the change. Thank you for the PR.

[pboettch]

Will this submodule be updated in the iot-sdk-c as well? Ideally before the next tag.

[villepalo]

@jebrando I'm afraid that the segfault occurs again after commit 304f381. Actuall the test case mqtt_client_dowork_does_nothing_if_disconnected_2 reveals that it will occur. Isn't so that the test case proves xio_dowork gets called even if client is disconnected? That is, xioHandle is not null and it may lead to a segfault.