Allow sv = &PL_sv_undef; sv_set_undef(sv) to work

RT #130385

Technically

    sv = &PL_sv_undef;
    ....
    sv_set_undef(sv)

is modifying a read-only variable and so should croak, but some XS code
relies on the behaviour previous to the introduction of sv_set_undef(),
where:

    sv = &PL_sv_undef;
    ....
    sv_setsv(sv, &PL_undef)

silently succeeds (sv_setsv() returns immediately if src and dst
addresses are the same).

sv.c

@@ -4808,8 +4808,13 @@ Perl_sv_set_undef(pTHX_ SV *sv)
 
     if (type <= SVt_IV) {
         assert(!SvGMAGICAL(sv));
-        if (SvREADONLY(sv))
+        if (SvREADONLY(sv)) {
+            /* does undeffing PL_sv_undef count as modifying a read-only
+             * variable? Some XS code does this */
+            if (sv == &PL_sv_undef)
+                return;
             Perl_croak_no_modify();
+        }
 
         if (SvROK(sv)) {
             if (SvWEAKREF(sv))