gwin32appinfo: Fix an incorrect size vs wide-length comparison

`value_size` is in bytes, whereas `ms_resource_prefix_len` is in wide
characters, so they cannot be compared directly. This meant that if
12 ≤ `value_size` < 24 then the call to `memcmp()` would read off the
end of `value`.

Fix it by using a wide-character and nul-aware comparison function and
operating only on wide-lengths. This is safe because
`g_win32_registry_key_get_value_w()` guarantees that string-typed return
values are nul-terminated.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3649

gio/gwin32appinfo.c

@@ -3669,10 +3669,7 @@ grab_registry_string (GWin32RegistryKey  *handler_appkey,
 
   /* There's no way for us to resolve "ms-resource:..." strings */
   if (value != NULL &&
-      value_size >= ms_resource_prefix_len &&
-      memcmp (value,
-              ms_resource_prefix,
-              ms_resource_prefix_len * sizeof (gunichar2)) == 0)
+      wcsncmp (value, ms_resource_prefix, ms_resource_prefix_len) == 0)
     g_clear_pointer (&value, g_free);
 
   if (value == NULL)