Integrate change #9108 from maintperl to mainline.

Integrate change #9108 from maintperl to mainline.

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-link: @9108 on //depot/maint-5.6/perl: 1cf1f64f42eb50a67f2427ff9d6d24023a2b9997

p4raw-id: //depot/perl@9109
p4raw-branched: from //depot/maint-5.6/perl@9107 'branch in'
	t/op/anonsub.t
p4raw-integrated: from //depot/maint-5.6/perl@9107 'merge in' sv.c
	(@8871..) embed.h (@8886..) pod/perlapi.pod proto.h (@8993..)
	embed.pl (@8995..) MANIFEST (@9030..) op.c op.h (@9055..)
	pp_ctl.c (@9076..)