[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(+)