Assertion failing in master, predicate.c

Hackers,

I stumbled upon an assertion while testing master for possible
bugs. I am reporting it here in the hope that this report will
be useful. The attached small regression test patch consistently
triggers an assert in predicate.c:

TRAP: FailedAssertion("!isCommit ||
SxactIsPrepared(MySerializableXact)", File: "predicate.c", Line: 3372)

I originally hit this from sources with less than recent
code checked out, but the error is the same in a recent,
fresh `git clone` (4a0aab14dcb35550b55e623a3c194442c5666084)
The problem does not reproduce for me in REL_12_STABLE, though the
same assertion does exist in that branch.

I built on my laptop:

Linux 4.19.0-5-amd64 #1 SMP Debian 4.19.37-3 (2019-05-15) x86_64
GNU/Linux

I built using

`./configure --enable-cassert --enable-tap-tests --with-perl
--with-python --with-tcl`

The perl, python, and tcl options don't appear to matter, as nothing
changes using

`./configure --enable-cassert && make -j4 && make check-world`

--
Mark Dilger