segfault on secure URI with no SSL options supplied

[fpagliughi]

Describe the bug

If a secure URI, like mqtts://... is supplied to the the serverURIs field of the MQTTAsync_connectOptions, but no SSL options given to the connect options (i.e. conn_opts.ssl == NULL), then the program segfaults.

To Reproduce

Take a sample app, like MQTTAsync_publish.c

Set the URI as an empty string in the call to create the client:

if ((rc = MQTTAsync_create(&client, "", CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL)) != MQTTASYNC_SUCCESS)
{ ...

The add a secure URI to the connect options without specifying SSL options, like:

char* const uris[] = {"mqtts://test.mosquitto.org:8883"};

conn_opts.serverURIcount = 1;
conn_opts.serverURIs = uris;

conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
conn_opts.onSuccess = onConnect;
conn_opts.onFailure = onConnectFailure;
conn_opts.context = client;

if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
{ ...

The call to MQTTAsync_connect() crashes with a segfault.

Expected behavior
Return -6 from the call to MQTTAsync_connect() in this case.

[fpagliughi]

Note that this was originally reported on the Paho Rust repo:
eclipse-paho/paho.mqtt.rust#238