fix memory leak in C<sub X { sub {} }> arising from a refcount

fix memory leak in C<sub X { sub {} }> arising from a refcount
loop between the outer sub and the inner prototype anonsub

this also enables closures returned by subroutines that
subsequently get redefined to work without generating coredumps :)

completely removed the free_closures() hack--it shouldn't be
needed anymore

p4raw-id: //depot/maint-5.6/perl@9108