Use proper casing of username in avatar filesystem setup

* before you could request an avatar for User instead of user
  which sets up the filesystem for that user twice causing
  the sharing codes collision detection to detect a lot of
  collisions

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

lib/private/AvatarManager.php

@@ -91,6 +91,8 @@ class AvatarManager implements IAvatarManager {
 		if (is_null($user)) {
 			throw new \Exception('user does not exist');
 		}
+		// sanitize userID - fixes casing issue (needed for the filesystem stuff that is done below)
+		$userId = $user->getUID();
 
 		/*
 		 * Fix for #22119