w32: Make qemu_vfree() accept NULL like the POSIX implementation

w32: Make qemu_vfree() accept NULL like the POSIX implementation

On POSIX, qemu_vfree() accepts NULL, because it's merely wrapper
around free().  As far as I can tell, the Windows implementation
doesn't.  Breeds bugs that bite only under Windows.

Make the Windows implementation behave like the POSIX implementation.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>