Give better error message if a client sends a password without a username.

Closes #1015. Thanks to TabascoEye.

ChangeLog.txt

@@ -7,6 +7,8 @@ Broker:
   have its DISCONNECT message processed properly and so no Will will be sent.
   Closes #7.
 - $SYS/broker/clients/disconnected should never be negative. Closes #287.
+- Give better error message if a client sends a password without a username.
+  Closes #1015.
 
 Library:
 - Fix memory leak that occurred if mosquitto_reconnect() was used when TLS

src/handle_connect.c

@@ -395,6 +395,7 @@ int handle__connect(struct mosquitto_db *db, struct mosquitto *context)
 		if(context->protocol == mosq_p_mqtt311){
 			if(password_flag){
 				/* username_flag == 0 && password_flag == 1 is forbidden */
+				log__printf(NULL, MOSQ_LOG_ERR, "Protocol error from %s: password without username, closing connection.", client_id);
 				rc = MOSQ_ERR_PROTOCOL;
 				goto handle_connect_error;
 			}