pgsql: Catch overflow when rounding intervals in AdjustIntervalForTypmo

Catch overflow when rounding intervals in AdjustIntervalForTypmod.

Previously, an interval microseconds field close to INT64_MAX or
INT64_MIN could overflow, producing a result with not even the
correct sign, while being rounded to match a precision specification.

This seems worth fixing, but not worth back-patching, in part
because the ereturn() notation doesn't exist very far back.

Report and patch by Joseph Koshakow (some cosmetic mods by me)

Discussion: https://postgr.es/m/CAAvxfHfpuLgqJYzkUcher466Z1LpmE+5Sm+zc8L6zKCOQ+6TDQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5ebc9c90173f32cffe373a80835f157b9ebfa3bd

Modified Files
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src/backend/utils/adt/timestamp.c | 22 ++++++++++++++--------
src/test/regress/expected/interval.out | 8 ++++++++
src/test/regress/sql/interval.sql | 2 ++
3 files changed, 24 insertions(+), 8 deletions(-)