be reset if its SV is freed. (see change 22688 (30952)).
A real live bug found by Slaven and Andreas whilst smoking maint-5.8.x.
I guess that we should audit the interpreter structure for any others.
p4raw-id: //depot/perl@34479
cv_undef((CV*)sv);
goto freescalar;
case SVt_PVHV:
+ if (PL_last_swash_hv == (HV*)sv) {
+ PL_last_swash_hv = NULL;
+ }
Perl_hv_kill_backrefs(aTHX_ (HV*)sv);
hv_undef((HV*)sv);
break;