Assert that backreferences array is not freed rather than handling it

In several places where the weakrefs backreferences array is used
or freed, the code checks whether the array has already been freed
and if so skips. Since the array already being freed is a bad bug,
lets instead assert that this never happens, based on the
assumptions that (a) such premature freeing bugs are likely ironed
out by now, (b) if they aren't then we want to know about them and
fix them rather than silently skip.

p4raw-id: //depot/perl@34210

sv.c

@@ -5086,14 +5086,11 @@ S_sv_del_backref(pTHX_ SV *const tsv, SV *const sv)
 	if (mg)
 	    av = (AV *)mg->mg_obj;
     }
-    if (!av) {
-	if (PL_in_clean_all)
-	    return;
+
+    if (!av)
 	Perl_croak(aTHX_ "panic: del_backref");
-    }
 
-    if (SvIS_FREED(av))
-	return;
+    assert(!SvIS_FREED(av));
 
     svp = AvARRAY(av);
     /* We shouldn't be in here more than once, but for paranoia reasons lets
@@ -5123,9 +5120,8 @@ Perl_sv_kill_backrefs(pTHX_ SV *const sv, AV *const av)
     PERL_ARGS_ASSERT_SV_KILL_BACKREFS;
     PERL_UNUSED_ARG(sv);
 
-    /* Not sure why the av can get freed ahead of its sv, but somehow it does
-       in ext/B/t/bytecode.t test 15 (involving print <DATA>)  */
-    if (svp && !SvIS_FREED(av)) {
+    assert(!svp || !SvIS_FREED(av));
+    if (svp) {
 	SV *const *const last = svp + AvFILLp(av);
 
 	while (svp <= last) {