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.