thinkpad_acpi: fix possible NULL pointer dereference if kstrdup failed

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>