Fix some memory leaks in the WAL receiver

Fix some memory leaks in the WAL receiver

These are old leaks, that can pile up if a WAL receiver stays alive,
waiting for new WAL data after the sender has switched to a new
timeline.

While this is technically a bug, the impact is minimal and would only
become noticeable if the WAL sender handles a lot of timeline switches,
so no backpatch is done.  Note that in most cases, primary_conninfo
would be updated in a standby to point to a new sender, meaning a
restart of the WAL receiver.  Let's be clean on HEAD, though.

Author: DaeMyung Kang <charsyam@gmail.com>
Discussion: https://postgr.es/m/20260426170100.847923-1-charsyam@gmail.com
Discussion: https://postgr.es/m/20260426170219.849330-1-charsyam@gmail.com