Fix memory leak in mosquitto_tls_opts_set()

When calling mosquitto_tls_opts_set() multiple time in a row
on the same mosquitto context (Example: trying to reconnect
to mqtt broker), some of the tls options are not free, resulting in a memory leak.

The TLS options (tls_version, tls_ciphers) must be free before being set.

Signed-off-by: Guillaume PELLEGRINO <guillaume.pellegrino@yahoo.com>

lib/options.c

@@ -228,19 +228,23 @@ int mosquitto_tls_opts_set(struct mosquitto *mosq, int cert_reqs, const char *tl
 				|| !strcasecmp(tls_version, "tlsv1.2")
 				|| !strcasecmp(tls_version, "tlsv1.1")){
 
+			mosquitto__free(mosq->tls_version);
 			mosq->tls_version = mosquitto__strdup(tls_version);
 			if(!mosq->tls_version) return MOSQ_ERR_NOMEM;
 		}else{
 			return MOSQ_ERR_INVAL;
 		}
 	}else{
+		mosquitto__free(mosq->tls_version);
 		mosq->tls_version = mosquitto__strdup("tlsv1.2");
 		if(!mosq->tls_version) return MOSQ_ERR_NOMEM;
 	}
 	if(ciphers){
+		mosquitto__free(mosq->tls_ciphers);
 		mosq->tls_ciphers = mosquitto__strdup(ciphers);
 		if(!mosq->tls_ciphers) return MOSQ_ERR_NOMEM;
 	}else{
+		mosquitto__free(mosq->tls_ciphers);
 		mosq->tls_ciphers = NULL;
 	}