Fix memory overread in registry.rb

	The terminator is not actually getting filled in; we're simply passing
	(two) bytes of empty memory as the NUL terminator. This can lead to
	garbage characters getting written to registry values.

	Fix this by explicitly putting a WCHAR_NUL character into the string to
	be sent to the registry API, like we do in the MULTI_SZ case.

	[Bug #20096]
	---
	 ext/win32/lib/win32/registry.rb | 7 ++-----
	 1 file changed, 2 insertions(+), 5 deletions(-)