Fix memory leak when copying ST tables

st_copy allocates a st_table, which is not needed for hashes since it is
allocated by VWA and embedded, so this causes a memory leak.

The following script demonstrates the issue:

```ruby
20.times do
  100_000.times do
    {a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8, i: 9}
  end

  puts `ps -o rss= -p #{$$}`
end
```

1 files changed, 5 insertions, 2 deletions

diff --git a/hash.c b/hash.c
index a74db258e4..a779c6f218 100644
--- a/hash.c
+++ b/hash.c
@@ -1482,7 +1482,9 @@ hash_copy(VALUE ret, VALUE hash)
     else {
         HASH_ASSERT(sizeof(st_table) <= sizeof(ar_table));
 
-        RHASH_ST_TABLE_SET(ret, st_copy(RHASH_ST_TABLE(hash)));
+        RHASH_SET_ST_FLAG(ret);
+        st_replace(RHASH_ST_TABLE(ret), RHASH_ST_TABLE(hash));
+
         rb_gc_writebarrier_remember(ret);
     }
     return ret;
@@ -1776,7 +1778,8 @@ rb_hash_s_create(int argc, VALUE *argv, VALUE klass)
             }
             else {
                 hash = hash_alloc(klass);
-                hash_copy(hash, tmp);
+                if (!RHASH_EMPTY_P(tmp))
+                    hash_copy(hash, tmp);
                 return hash;
             }
         }