pp_aassign: remove panic check under PERL_RC_STACK

There is some code in S_aassign_copy_common() (helper function for
pp_aassign()) which checks for SvIS_FREED() of the current arg.
The comments imply that the check is there specifically to help a TODO
test in t/op/sort.t not randomly crash. That test does stuff which
triggers a premature free of args on the stack.

This test was later disabled entirely, as whatever panics or crashes
were being triggered weren't being caught by the eval {} it was wrapped
in.

This commit, only on PERL_RC_STACK builds, skips the SvIS_FREED check,
and re-enables the test in sort.t (as a real test, not a TODO).

pp_hot.c

@@ -2493,13 +2493,20 @@ S_aassign_copy_common(pTHX_ SV **firstlelem, SV **lastlelem,
 
             TAINT_NOT;	/* Each item is independent */
 
+#ifndef PERL_RC_STACK
+            /* The TODO test was eventually commented out. It's now been
+             * revived, but only on PERL_RC_STACK builds. Continue
+             * this hacky workaround otherwise - DAPM Sept 2023 */
+
             /* Dear TODO test in t/op/sort.t, I love you.
                (It's relying on a panic, not a "semi-panic" from newSVsv()
                and then an assertion failure below.)  */
             if (UNLIKELY(SvIS_FREED(svr))) {
                 Perl_croak(aTHX_ "panic: attempt to copy freed scalar %p",
                            (void*)svr);
             }
+#endif
+
             /* avoid break flag while copying; otherwise COW etc
              * disabled... */
             SvFLAGS(svr) &= ~SVf_BREAK;

t/op/sort.t

@@ -7,7 +7,7 @@ BEGIN {
     set_up_inc('../lib');
 }
 use warnings;
-plan(tests => 204);
+plan(tests => 205);
 use Tie::Array; # we need to test sorting tied arrays
 
 # these shouldn't hang
@@ -884,18 +884,14 @@ cmp_ok($answer,'eq','good','sort subr called from other package');
     }
 }
 
-
-
-# I commented out this TODO test because messing with FREEd scalars on the
-# stack can have all sorts of strange side-effects, not made safe by eval
-# - DAPM.
-#
-#{
-#    local $TODO = "sort should make sure elements are not freed in the sort block";
-#    eval { @nomodify_x=(1..8);
-#	   our @copy = sort { undef @nomodify_x; 1 } (@nomodify_x, 3); };
-#    is($@, "");
-#}
+SKIP:
+{
+    skip "freed args not under PERL_RC_STACK", 1
+        unless (Internals::stack_refcounted() & 1);
+    eval { @nomodify_x=(1..8);
+	   our @copy = sort { undef @nomodify_x; 1 } (@nomodify_x, 3); };
+    is($@, "");
+}
 
 
 # Sorting shouldn't increase the refcount of a sub