Don't propagate a null subtransaction snapshot up to parent transaction.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 71dca408c0030ad76044c6b17367c9fbeac511ec
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2016-08-07T17:15:55Z
Releases: 9.5.4
Don't propagate a null subtransaction snapshot up to parent transaction.

This oversight could cause logical decoding to fail to decode an outer
transaction containing changes, if a subtransaction had an XID but no
actual changes.  Per bug #14279 from Marko Tiikkaja.  Patch by Marko
based on analysis by Andrew Gierth.

Discussion: <20160804191757.1430.39011@wrigleys.postgresql.org>

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