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