osdep/language-win: fix use of GetUserPreferredUILanguages()

Commit baf2fd2 fixed this for 2nd call
to GetSystemPreferredUILanguages(), but apparently regressed the 1st
call, duh.

To retrieve the size both buffer has to be NULL and size 0.

Memory has a lot of zeros, because it has been working quite
consistently on CI.

Author: kasper93

osdep/language-win.c

@@ -39,7 +39,7 @@ char **mp_get_user_langs(void)
     char **ret = NULL;
     size_t ret_count = 0;
     wchar_t *buf = NULL;
-    ULONG size, count;
+    ULONG size = 0, count;
 
     if (!GetUserPreferredUILanguages(MUI_LANGUAGE_NAME, &count, NULL, &size))
         goto done;