pgsql: Use SvROK(sv) rather than directly checking SvTYPE(sv) == SVt_RV

Log Message:
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Use SvROK(sv) rather than directly checking SvTYPE(sv) == SVt_RV in plperl.
The latter is considered unwarranted chumminess with the implementation,
and can lead to crashes with recent Perl versions.

Report and fix by Tim Bunce.  Back-patch to all versions containing the
questionable coding pattern.

Tags:
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REL8_3_STABLE

Modified Files:
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    pgsql/src/pl/plperl:
        plperl.c (r1.136.2.7 -> r1.136.2.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c?r1=1.136.2.7&r2=1.136.2.8)