advapi32: Initialize retkey value in open_key.

Office 2013 registry virtualization depends on this. Its NtOpenKey[Ex]
hook doesn't set it to NULL, but it's expected to be NULL.

Signed-off-by: Jacek Caban <jacek@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/advapi32/registry.c

@@ -212,6 +212,8 @@ static NTSTATUS open_key( HKEY *retkey, DWORD options, ACCESS_MASK access, OBJEC
     DWORD pos = 0, i = 0, len = attr->ObjectName->Length / sizeof(WCHAR);
     UNICODE_STRING str;
 
+    *retkey = NULL;
+
     if (!force_wow32)
     {
         if (options & REG_OPTION_OPEN_LINK) attr->Attributes |= OBJ_OPENLINK;