[PATCH] Clear walsender reader pointers after cleanup

Hi,

FreeDecodingContext() frees the reader when logical streaming ends, but
xlogreader is left pointing to it. If the next command on that connection
fails, WalSndErrorCleanup() still follows that pointer.

I tried sending CopyDone, draining the replies, then TIMELINE_HISTORY 99.
With --enable-cassert, I get close(0x7F7F7F7F). The connection stays up,
but the attached patch stops that invalid close.

The patch clears the pointers after streaming and error cleanup. For
physical streaming, I've also freed the reader before returning. Is there
a reason to leave that part to cmd_context reset instead?

I've adjusted the assertion too, since it runs after the streaming
function returns. At that point the pointers are now NULL.

Regards,
Ayush