Set minimum keepalive argument to `mosquitto_connect*()` to be 5 seconds.

Closes #1550. Thanks to Markus Gothe.

ChangeLog.txt

@@ -9,6 +9,10 @@ Broker:
   listening socket, by calling freeaddrinfo. Closes #1565.
 - Fix config->user not being freed on exit. Closes #1564.
 
+Library:
+- Set minimum keepalive argument to `mosquitto_connect*()` to be 5 seconds.
+  Closes #1550.
+
 
 1.6.8 - 20191128
 ================

lib/connect.c

@@ -43,6 +43,7 @@ static int mosquitto__connect_init(struct mosquitto *mosq, const char *host, int
 
 	if(!mosq) return MOSQ_ERR_INVAL;
 	if(!host || port <= 0) return MOSQ_ERR_INVAL;
+	if(keepalive < 5) return MOSQ_ERR_INVAL;
 
 	if(mosq->id == NULL && (mosq->protocol == mosq_p_mqtt31 || mosq->protocol == mosq_p_mqtt311)){
 		mosq->id = (char *)mosquitto__calloc(24, sizeof(char));