[perl #128532] Crash vivifying stub in deleted pkg

v5.17.0-515-g186a5ba, which added newSTUB, did not take into account
that a GV may have a null GvSTASH pointer, if its stash has been
freed, so this crashes:

delete $My::{"Foo::"}; \&My::Foo::foo

Author: Father Chrysostomos

op.c

@@ -9603,7 +9603,7 @@ Perl_newSTUB(pTHX_ GV *gv, bool fake)
     assert(!GvCVu(gv));
     GvCV_set(gv, cv);
     GvCVGEN(gv) = 0;
-    if (!fake && HvENAME_HEK(GvSTASH(gv)))
+    if (!fake && GvSTASH(gv) && HvENAME_HEK(GvSTASH(gv)))
 	gv_method_changed(gv);
     if (SvFAKE(gv)) {
 	cvgv = gv_fetchsv((SV *)gv, GV_ADDMULTI, SVt_PVCV);

t/op/ref.t

@@ -8,7 +8,7 @@ BEGIN {
 
 use strict qw(refs subs);
 
-plan(235);
+plan(236);
 
 # Test this first before we extend the stack with other operations.
 # This caused an asan failure due to a bad write past the end of the stack.
@@ -124,6 +124,10 @@ is (join(':',@{$spring2{"foo"}}), "1:2:3:4");
     is ($called, 1);
 }
 is ref eval {\&{""}}, "CODE", 'reference to &{""} [perl #94476]';
+delete $My::{"Foo::"}; 
+is ref \&My::Foo::foo, "CODE",
+  'creating stub with \&deleted_stash::foo [perl #128532]';
+
 
 # Test references to return values of operators (TARGs/PADTMPs)
 {