pgsql: Allow buffer tuple table slots to materialize after ExecStoreVir

Allow buffer tuple table slots to materialize after ExecStoreVirtualTuple().

While not common, it can be useful to store a virtual tuple into a
buffer tuple table slot, and then materialize that slot. So far we've
asserted out, which surprisingly wasn't a problem for anything in
core. But that seems fragile, and it also breaks redis_fdw after
ff11e7f4b9.

Thus, allow materializing a virtual tuple stored in a buffer tuple
table slot.

Author: Andres Freund
Discussion:
https://postgr.es/m/20190227181621.xholonj7ff7ohxsg@alap3.anarazel.de
https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f414abd62da7aa3cfd75e9047affd94c029c7c04

Modified Files
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src/backend/executor/execTuples.c | 48 +++++++++++++++++++++++++--------------
1 file changed, 31 insertions(+), 17 deletions(-)