Fix memory leak in Hash#compare_by_identity

We didn't free the old ST before overwriting it which caused a leak.
Found with RUBY_FREE_ON_EXIT.

Co-authored-by: Peter Zhu <peter@peterzhu.ca>

hash.c

@@ -4372,6 +4372,7 @@ rb_hash_compare_by_id(VALUE hash)
 
     rb_hash_foreach(hash, rb_hash_rehash_i, (VALUE)tmp);
 
+    rb_hash_free(hash);
     RHASH_ST_TABLE_SET(hash, identtable);
     RHASH_ST_CLEAR(tmp);
 

test/ruby/test_hash.rb

@@ -1909,6 +1909,14 @@ def test_st_literal_memory_leak
     end;
   end
 
+  def test_compare_by_id_memory_leak
+    assert_no_memory_leak([], "", <<~RUBY, rss: true)
+      1_000_000.times do
+        {a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8}.compare_by_identity
+      end
+    RUBY
+  end
+
   def test_try_convert
     assert_equal({1=>2}, Hash.try_convert({1=>2}))
     assert_equal(nil, Hash.try_convert("1=>2"))