For Win32_PhysicalMemory PartNumber and Serial properties.

Injustice 2 calls SysStringLen directly on the value without checking
its type.
Signed-off-by: Rémi Bernon <rbernon@codeweavers.com>
Signed-off-by: Hans Leidekker <hans@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>