pgsql: Fix recently added code for SQL years-months interval syntax so

Log Message:
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Fix recently added code for SQL years-months interval syntax so that
it behaves correctly for a leading minus sign, zero year value, and
nonzero month value.  Per discussion with Ron Mayer.

Modified Files:
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    pgsql/src/backend/utils/adt:
        datetime.c (r1.195 -> r1.196)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.195&r2=1.196)