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

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 3a779366025504d6993e6a3b1281cd114a0abb71
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2013-03-04T20:14:00Z
Releases: 9.1.9
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.

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