Fix MQTT v5 clients not being able to specify a password without a us…

…ername.

Thanks to Erik Moqvist.

Closes #1274.

client/client_shared.c

@@ -1081,8 +1081,8 @@ int client_opts_set(struct mosquitto *mosq, struct mosq_config *cfg)
  }
  cfg->will_props = NULL;
 
- if(cfg->username && mosquitto_username_pw_set(mosq, cfg->username, cfg->password)){
- err_printf(cfg, "Error: Problem setting username and password.\n");
+ if((cfg->username || cfg->password) && mosquitto_username_pw_set(mosq, cfg->username, cfg->password)){
+ err_printf(cfg, "Error: Problem setting username and/or password.\n");
  mosquitto_lib_cleanup();
  return 1;
  }