Sanitize File Path

The function below builds a file path by concatenating a base directory with user-supplied input.

The bug: an attacker can pass "../../../etc/passwd" as the filename and read any file on the server — a classic path traversal attack.

Your task: fix solve(base_path, user_input) so that:

• Valid filenames within the base directory return the full, normalized path.
• Any input that would resolve outside base_path returns None.

solve('/uploads', 'photo.jpg')         # → '/uploads/photo.jpg'
solve('/uploads', 'sub/image.png')     # → '/uploads/sub/image.png'
solve('/uploads', '../etc/passwd')     # → None
solve('/uploads', '../../secret.key')  # → None