pgsql: Prevent spurious "indexes on virtual generated columns are not s

Prevent spurious "indexes on virtual generated columns are not supported".

Both of the checks in DefineIndex() that can produce this error
message have a guard against negative attribute numbers, but lack a
guard to ensure that attno is non-zero. As a result, we can index
off the beginning of the TupleDesc and read a garbage byte for
attgenerated. If that byte happens to be 'v', we'll incorrectly
produce the error mentioned above.

The first call site is easy to hit: any attempt to create an
expression index does so. The second one is not currently hit in
the regression tests, but can be hit by something like
CREATE INDEX ON some_table ((some_function(some_table))).

Found by study of a test_plan_advice failure on buildfarm member
skink, though this issue has nothing to do with test_plan_advice
and seems to have only been revealed by happenstance.

Backpatch-through: 18
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: http://postgr.es/m/CA+TgmoacixUZVvi00hOjk_d9B4iYKswWP1gNqQ8Vfray-AcOCA@mail.gmail.com

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/cceb9c18a50b3611537050b3f9bb4685072823b4

Modified Files
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src/backend/commands/indexcmds.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)