pgsql: Avoid integer overflow hazard in interval_time().

Avoid integer overflow hazard in interval_time().

When casting an interval to a time, the original code suffered from
64-bit integer overflow for inputs with a sufficiently large negative
"time" field, leading to bogus results.

Fix by rewriting the algorithm in a simpler form, that more obviously
cannot overflow. While at it, improve the test coverage to include
negative interval inputs.

Discussion: https://postgr.es/m/CAEZATCXoUKHkcuq4q63hkiPsKZJd0kZWzgKtU%2BNT0aU4wbf_Pw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3850d4dec1d91c4fdce274f42986840444d5593e

Modified Files
--------------
src/backend/utils/adt/date.c | 15 +++------------
src/test/regress/expected/horology.out | 12 ++++++++++++
src/test/regress/sql/horology.sql | 2 ++
3 files changed, 17 insertions(+), 12 deletions(-)