Check values passed back from PLPerl to the database, via function return,

Check values passed back from PLPerl to the database, via function return,
trigger tuple modification or SPI call, to ensure they are valid in the
server encoding. Along the way, replace uses of SvPV(foo, PL_na)
with SvPV_nolen(foo) as recommended in the perl docs. Bug report from
Hannu Krosing.