SHARED string is not required when sharing POOL string

The heap string buffer of POOL string always exists, does not need to be
released, and read only, so it can be shared as NOFREE string.

src/string.c

@@ -526,7 +526,7 @@ str_make_shared(mrb_state *mrb, struct RString *orig, struct RString *s)
   size_t len = (size_t)orig->as.heap.len;
 
   mrb_assert(!RSTR_EMBED_P(orig));
-  if (RSTR_NOFREE_P(orig)) {
+  if (RSTR_NOFREE_P(orig) || RSTR_POOL_P(orig)) {
     str_init_nofree(s, orig->as.heap.ptr, len);
   }
   else if (RSTR_SHARED_P(orig)) {
@@ -535,7 +535,7 @@ str_make_shared(mrb_state *mrb, struct RString *orig, struct RString *s)
   else if (RSTR_FSHARED_P(orig)) {
     str_init_fshared(orig, s, orig->as.heap.aux.fshared);
   }
-  else if (mrb_frozen_p(orig) && !RSTR_POOL_P(orig)) {
+  else if (mrb_frozen_p(orig)) {
     str_init_fshared(orig, s, orig);
   }
   else {