Fixes a regression introduced by d5410870.

AssociateFocus does not grab ownership of the Document Manager, but it does
return it in the "prev" output parameter with an increased refcount. So,
if the docmgr gets destroyed and AssociateFocus is called again, its "prev"
parameter will refer to freed memory. Windows sets it to NULL in such case.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=48469Signed-off-by: Gabriel Ivăncescu <gabrielopcode@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>