[vim/vim] u_read_undo() leaks the file name when the undo file owner differs (PR #20987)

## Problem

When the owner of an undo file differs from the owner of the text file and
the current user, u_read_undo() returns without freeing the file name it
allocated with u_get_undo_file_name().  Every other exit frees it under
the "theend" label; this early return sits before the file pointer is
initialized, so it cannot use that label.

## Solution

Free the file name before returning.

Noticed while reworking #20942; independent of that change, so sent
separately.

AI assistance is acknowledged with Co-Authored-By trailers on the commit,
per AGENTS.md.

You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20987

-- Commit Summary --

  * u_read_undo() leaks the file name when the undo file owner differs

-- File Changes --

    M src/undo.c (1)

-- Patch Links --

https://github.com/vim/vim/pull/20987.patch
https://github.com/vim/vim/pull/20987.diff

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