Fix memory leak in tee_svc_cryp_derive_key

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (QEMU virt)
That's funny because that's what I had written at first, then I removed the "= NULL" thinking "that's useless" ;-)
OK I agree and I will fix that.