Fix double free on exit in mosquitto_pub.

Closes #1280. Thanks to Lucky Saini.

ChangeLog.txt

@@ -43,6 +43,9 @@ Clients:
 - Fix MQTT v5 clients not being able to specify a password without a username.
   Closes #1274.
 
+Clients:
+- Fix double free on exit in mosquitto_pub. Closes #1280.
+
 Documentation:
 - Remove references to Python binding and C++ wrapper in libmosquitto man
   page. Closes #1266.

client/pub_client.c

@@ -508,6 +508,7 @@ int main(int argc, char *argv[])
 
 	if(cfg.message && cfg.pub_mode == MSGMODE_FILE){
 		free(cfg.message);
+		cfg.message = NULL;
 	}
 	mosquitto_destroy(mosq);
 	mosquitto_lib_cleanup();