my_plvarsp nulling and PERL_GLOBAL_STRUCT_PRIVATE

With PERL_GLOBAL_STRUCT_PRIVATE, all "global" vars are in a malloc()d
structure pointed to by the static var my_plvarsp. At exit, this struct is
freed and my_plvarsp is set to NULL.

My previous commit c1181d2b skipped the free if PL_veto_cleanup is set
(as it would be if other threads are still running for example), but
still left my_plvarsp getting set to NULL. Thus other threads could still
deref a null pointer if they accessed a "global" var just as the main
thread was exiting.

This commit makes the veto skip the NULLing in addition to the freeing.

This commit is quite late into the code freeze, but it's a follow-up to the
earlier attempt to get smokes not to fail, and all the affected code is
within #ifdef PERL_GLOBAL_STRUCT_PRIVATE, so it shouldn't affect
mainstream builds at all. (Famous last words.)