pgsql: Fix ginint4_queryextract() to actually do what it was intended to

Log Message:
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Fix ginint4_queryextract() to actually do what it was intended to do for an
unsatisfiable query, such as indexcol && empty_array.  It should return -1
to tell GIN no scan is required; but silly typo disabled the logic for that,
resulting in unnecessary "GIN indexes do not support whole-index scans" error.
Per bug report from Jeff Trout.

Back-patch to 8.3 where the logic was introduced.

Tags:
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REL8_4_STABLE

Modified Files:
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    pgsql/contrib/intarray:
        _int_gin.c (r1.10 -> r1.10.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/intarray/_int_gin.c?r1=1.10&r2=1.10.2.1)