merge revision(s) c6f84e918943a0bf8db6fee556fc53180d257510: [Backport #19398]

	[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
	```
	---
	 gc.c                      | 1 +
	 test/ruby/test_weakmap.rb | 9 +++++++++
	 2 files changed, 10 insertions(+)

1 files changed, 1 insertions, 0 deletions

diff --git a/gc.c b/gc.c
index cb33eb511b..a9d7352f2f 100644
--- a/gc.c
+++ b/gc.c
@@ -11987,6 +11987,7 @@ 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