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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3ff660bbeb96086cb1cf880bfb4e2e350cbd21b2

Modified Files
--------------
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(-)