pgsql: Fix null-pointer crash in postgres_fdw's conversion_error_callba

Fix null-pointer crash in postgres_fdw's conversion_error_callback.

Commit c7b7311f6 adjusted conversion_error_callback to always use
information from the query's rangetable, to avoid doing catalog lookups
in an already-failed transaction.  However, as a result of the utterly
inadequate documentation for make_tuple_from_result_row, I failed to
realize that fsstate could be NULL in some contexts.  That led to a
crash if we got a conversion error in such a context.  Fix by falling
back to the previous coding when fsstate is NULL.  Improve the
commentary, too.

Per report from Andrey Borodin.  Back-patch to 9.6, like the previous
patch.

Discussion: https://postgr.es/m/08916396-55E4-4D68-AB3A-BD6066F9E5C0@yandex-team.ru

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/36c9f7d96e1c95d45c7286a9355011289ab6c087

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out |   3 +
contrib/postgres_fdw/postgres_fdw.c            | 118 ++++++++++++++++---------
contrib/postgres_fdw/sql/postgres_fdw.sql      |   1 +
3 files changed, 82 insertions(+), 40 deletions(-)