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

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