Suppress compiler warning in new jsonb_plperl code.

Suppress compiler warning in new jsonb_plperl code.

Some compilers are evidently pickier than others about whether Perl's
I32 typedef should be considered equivalent to int.  Dodge the problem
by using a separate variable; the prior coding was a bit confusing anyway.

Per buildfarm.  Note this does nothing to fix the test failures due to
SV_to_JsonbValue not covering enough variable types.