Fix walsender error cleanup code

In commit 850196b610d2 I (Álvaro) failed to handle the case of walsender
shutting down on an error before setting up its 'xlogreader' pointer;
the error handling code dereferences the pointer, causing a crash.
Fix by testing the pointer before trying to dereference it.

Kyotaro authored the code fix; I adopted Nathan's test case to be used
by the TAP tests and added the necessary PostgresNode change.

Reported-by: Nathan Bossart <[email protected]>
Author: Kyotaro Horiguchi <[email protected]>
Author: Álvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/[email protected]

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