Write a Python program to simulate a buffer overflow, then implement input validation to prevent it. There are two parts to this assignment:

1) The program should display a welcome message and prompt the user for a username. Create a buffer overflow condition by allowing a user to input more data than the size of the allocated memory (causing the program to crash).

2) Implement input validation to mitigate the overflow vulnerability. Check that the username entered has a minimum length of 8 characters and a maximum of 25 characters. If the user enters a username length outside those limits, return an error message and prompt the user to re-enter the username.