[Bug #19398] Memory leak in WeakMap

There's a memory leak in ObjectSpace::WeakMap due to not freeing
the `struct weakmap`. It can be seen in the following script:

```
100.times do
  10000.times do
    ObjectSpace::WeakMap.new
  end

  # Output the Resident Set Size (memory usage, in KB) of the current Ruby process
  puts `ps -o rss= -p #{$$}`
end
```

1 files changed, 9 insertions, 0 deletions

diff --git a/test/ruby/test_weakmap.rb b/test/ruby/test_weakmap.rb
index 4c91661f86..86550cc221 100644
--- a/test/ruby/test_weakmap.rb
+++ b/test/ruby/test_weakmap.rb
@@ -167,4 +167,13 @@ class TestWeakMap < Test::Unit::TestCase
     assert_nothing_raised(FrozenError) {@wm[o] = 'foo'}
     assert_nothing_raised(FrozenError) {@wm['foo'] = o}
   end
+
+  def test_no_memory_leak
+    assert_no_memory_leak([], '', "#{<<~"begin;"}\n#{<<~'end;'}", "[Bug #19398]", rss: true, limit: 1.5, timeout: 60)
+    begin;
+      1_000_000.times do
+        ObjectSpace::WeakMap.new
+      end
+    end;
+  end
 end