Re: pg13: xlogreader API adjust

I think I've discovered a problem with 850196b6.  The following steps
can be used to trigger a segfault:

        # wal_level = logical
        psql postgres -c "create database testdb;"
        psql testdb -c "select pg_create_logical_replication_slot('slot', 'test_decoding');"
        psql "dbname=postgres replication=database" -c "START_REPLICATION SLOT slot LOGICAL 0/0;"

From a quick glance, I think the problem starts in
StartLogicalReplication() in walsender.c.  The call to
CreateDecodingContext() may ERROR before xlogreader is initialized in
the next line, so the subsequent call to WalSndErrorCleanup()
segfaults when it attempts to access xlogreader.

Nathan