pgsql: Detect more overflows in timestamp[tz]_pl_interval.

Detect more overflows in timestamp[tz]_pl_interval.

In commit 25cd2d640 I (tgl) opined that "The additions of the months
and microseconds fields could also overflow, of course.  However,
I believe we need no additional checks there; the existing range
checks should catch such cases".  This is demonstrably wrong however
for the microseconds field, and given that discovery it seems prudent
to be paranoid about the months addition as well.

Report and patch by Joseph Koshakow.  As before, back-patch to all
supported branches.  (However, the test case doesn't work before
v15 because we didn't allow wider-than-int32 numbers in interval
literals.  A variant test could probably be built that fits within
that restriction, but it didn't seem worth the trouble.)

Discussion: https://postgr.es/m/CAAvxfHf77sRHKoEzUw9_cMYSpbpNS2C+J_+8Dq4+0oi8iKopeA@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3752e3d210287787a881ae3c68da019aa86f414e

Modified Files
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src/backend/utils/adt/timestamp.c      | 20 ++++++++++++++++----
src/test/regress/expected/horology.out |  4 ++++
src/test/regress/sql/horology.sql      |  2 ++
3 files changed, 22 insertions(+), 4 deletions(-)