SV's can be NULL (shit happens) (fixes RT86217)

Though it is not recommendable,  and VERY hard to do from a pure-perl
api, it isn't that hard for XS to set SV slots to NULL. In some cases
this can be measurable faster than using PL_sv_undef or similar.
Merge pull request #2 from Tux/master

SV's can be NULL (shit happens) (fixes RT86217)