Guard against use-after-free.

Coverity Scan 1431131, 1431135, 1431141.

client/sub_client_output.c

@@ -196,6 +196,8 @@ static int json_print_properties(cJSON *root, const mosquitto_property *properti
 				}
 				cJSON_AddItemToObject(user_json, strname, tmp);
 				free(strname);
+				strname = NULL;
+				strvalue = NULL;
 				tmp = NULL; /* Don't add this to prop_json below */
 				break;
 		}

lib/property_mosq.c

@@ -1092,6 +1092,8 @@ const mosquitto_property *mosquitto_property_read_binary(const mosquitto_propert
 	const mosquitto_property *p;
 	if(!proplist || (value && !len) || (!value && len)) return NULL;
 
+	if(value) *value = NULL;
+
 	p = property__get_property(proplist, identifier, skip_first);
 	if(!p) return NULL;
 	if(p->identifier != MQTT_PROP_CORRELATION_DATA
@@ -1146,6 +1148,9 @@ const mosquitto_property *mosquitto_property_read_string_pair(const mosquitto_pr
 	const mosquitto_property *p;
 	if(!proplist) return NULL;
 
+	if(name) *name = NULL;
+	if(value) *value = NULL;
+
 	p = property__get_property(proplist, identifier, skip_first);
 	if(!p) return NULL;
 	if(p->identifier != MQTT_PROP_USER_PROPERTY) return NULL;