pgsql: Fix overflow check in tm2timestamp (this time for sure).

Fix overflow check in tm2timestamp (this time for sure).

I fixed this code back in commit 841b4a2d5, but didn't think carefully
enough about the behavior near zero, which meant it improperly rejected
1999-12-31 24:00:00.  Per report from Magnus Hagander.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fa852303f0cda557e620af775c2e96b56eb365ba

Modified Files
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src/backend/utils/adt/timestamp.c          |    5 +++--
src/interfaces/ecpg/pgtypeslib/timestamp.c |    5 +++--
2 files changed, 6 insertions(+), 4 deletions(-)