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)
This works for now, but it would have felt better to initialize publicvalue and sharedsecret to NULL in case someone does a "goto out" too early.