pgsql: Fix portability issues in recently added make_timestamp/make_int

Fix portability issues in recently added make_timestamp/make_interval code.

Explicitly reject infinity/NaN inputs, rather than just assuming that
something else will do it for us.  Per buildfarm.

While at it, make some over-parenthesized and under-legible code
more readable.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f1ba94bcd9717b94b36868d6905547e313f3a359

Modified Files
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src/backend/utils/adt/timestamp.c |   32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)