pgsql: Fix core dump in transformValuesClause when there are no columns

Fix core dump in transformValuesClause when there are no columns.

The parser code that transformed VALUES from row-oriented to
column-oriented lists failed if there were zero columns.
You can't write that straightforwardly (though probably you
should be able to), but the case can be reached by expanding
a "tab.*" reference to a zero-column table.

Per bug #17477 from Wang Ke.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/17477-0af3c6ac6b0a6ae0@postgresql.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/fe20afaee8aac7838ed6e4a76baa83e547629582

Modified Files
--------------
src/backend/parser/analyze.c         | 18 ++++++------------
src/test/regress/expected/select.out |  7 +++++++
src/test/regress/sql/select.sql      |  5 +++++
3 files changed, 18 insertions(+), 12 deletions(-)