x86: debug Store - call kfree if only we really need it

We should call for kfree if only we really need it.
Though it's safe to call kfree with NULL pointer passed
in this code we've already tested the pointer and can
eliminate the call

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>