[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, 1 insertions, 0 deletions

diff --git a/gc.c b/gc.c
index 269d46daef..3baed251f4 100644
--- a/gc.c
+++ b/gc.c
@@ -12870,6 +12870,7 @@ wmap_free(void *ptr)
     st_foreach(w->obj2wmap, wmap_free_map, 0);
     st_free_table(w->obj2wmap);
     st_free_table(w->wmap2obj);
+    xfree(w);
 }
 
 static int