fix for [perl #41138] $_ leaks under threads

It's possible for an interpreter to get cloned with an SV copied that ends
up only linked from @_. For example, local $x causes a link to the
original $x SV to be added to the save stack, but when cloning a thread
the save stack isn't copied. If the old $x was also in someone's @_,
then it gets copied, but because @_'s elements  aren't normally reference
counted, old $x ends up with a refcount of zero, and you get a "leaked"
warning when the thread exits.

The workaround is to reify any reify-able AVs in the cloned interpreter
during cloning.

Also fixes [perl #70602], [perl #70974]

sv.c

@@ -11104,6 +11104,11 @@ Perl_sv_dup(pTHX_ const SV *const sstr, CLONE_PARAMS *const param)
 		    else {
 			while (items-- > 0)
 			    *dst_ary++ = sv_dup(*src_ary++, param);
+			if (!(param->flags & CLONEf_COPY_STACKS)
+			     && AvREIFY(sstr))
+			{
+			    av_reify(MUTABLE_AV(dstr)); /* #41138 */
+			}
 		    }
 		    items = AvMAX((const AV *)sstr) - AvFILLp((const AV *)sstr);
 		    while (items-- > 0) {

t/op/threads.t

@@ -129,6 +129,8 @@ foreach my $BLOCK (qw(CHECK INIT)) {
 EOI
 }
 
+} # TODO
+
 # Scalars leaked: 1
 fresh_perl_is(<<'EOI', 'ok', { }, 'Bug #41138');
     use threads;
@@ -141,7 +143,6 @@ fresh_perl_is(<<'EOI', 'ok', { }, 'Bug #41138');
     print 'ok';
 EOI
 
-} # TODO
 
 # [perl #45053] Memory corruption with heavy module loading in threads
 #