pgsql: Fix off-by-one error in PGTYPEStimestamp_fmt_asc

Fix off-by-one error in PGTYPEStimestamp_fmt_asc

When using %b or %B patterns to format a date, the code was simply using
tm_mon as an index into array of month names. But that is wrong, because
tm_mon is 1-based, while array indexes are 0-based. The result is we
either use name of the next month, or a segfault (for December).

Fix by subtracting 1 from tm_mon for both patterns, and add a regression
test triggering the issue. Backpatch to all supported versions (the bug
is there far longer, since at least 2003).

Reported-by: Paul Spencer
Backpatch-through: 9.4
Discussion: https://postgr.es/m/16143-0d861eb8688d3fef%40postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c59414da7bd3be9f80e7585fc173d21942468d3b

Modified Files
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src/interfaces/ecpg/pgtypeslib/timestamp.c         |  4 ++--
.../ecpg/test/expected/pgtypeslib-dt_test.c        | 24 ++++++++++++++++++----
.../ecpg/test/expected/pgtypeslib-dt_test.stderr   |  2 +-
.../ecpg/test/expected/pgtypeslib-dt_test.stdout   |  2 ++
src/interfaces/ecpg/test/pgtypeslib/dt_test.pgc    | 16 +++++++++++++++
5 files changed, 41 insertions(+), 7 deletions(-)